To investigate the expression of B7 homolog 4 (B7-H4) and its clinical significance in endometrial cancer. A total of 833 patients with endometrial cancer admitted to Peking Union Medical College Hospital, Chinese Academy of Medical Sciences from 2009 to 2019, were enrolled. The expression of B7-H4, mismatch repair (MMR), p53, programmed cell death ligand 1 (PD-L1) protein, and CD T lymphocyte density in endometrial cancer tissues were detected by the EnVision two-step method of immunohistochemical staining. First-generation sequencing (Sanger method) was used to determine molecular subtyping of endometrial cancer. The test was used to compare the differences in positive expression rate of B7-H4 protein in endometrial cancer tissues with different clinicopathological features and molecular subtyping, PD-L1 protein expression, and CD T lymphocyte density. Survival analyses [including recurrence-free survival (RFS) and disease-specific survival (DSS)] were performed for 664 patients with follow-up time≥3 months, with a median follow-up time of 31 months (range: 4-121 months), and the Cox proportional hazards regression model was used to analyze the relevant factors affecting the prognosis of patients with endometrial cancer. (1) The median age of 833 patients was 58 years (range: 25-88 years); pathological type: 595 with endometrioid carcinoma, 238 with non-endometrioid carcinoma; surgical-pathological staging: 542 cases at stage Ⅰ, 38 cases at stage Ⅱ, 173 cases at stage Ⅲ, and 45 cases at stage Ⅳ. Molecular subtyping was performed in 590 patients, including 50 with POLE mutation, 163 with mismatch repair defect (MMR-d) type, 246 with nospecific molecular change (NSMP) type, and 131 with p53 mutation subtype. (2) B7-H4 protein was expressed with brownish-yellow stainind in the cell membrane and cytoplasm of endometrial carcinoma, and the positivity rate of B7-H4 protein was 71.5% (596/833). The positivity rates of B7-H4 protein among patients with different age, surgical-pathological stage, tumor grade, pathological type, depth of muscular invasion, presence or absence of lymphovascular space invasion, and molecular subtype were significantly different (all <0.05). The positivity rates of B7-H4 protein among patients with different PD-L1 protein expression and CD T lymphocyte density were not significantly different (>0.05). The 5-year RFS (83.9%) and DSS (87.3%) of B7-H4 protein-positive patients had an increasing trend compared with the 5-year RFS (77.2%) and DSS (78.1%) of B7-H4 protein-negative patients, but there were not statistically significant differences (=0.053, =0.083). (3) Univariate analysis showed that the 5-year RFS and DSS of patients with different age, tumor grade, surgical-pathological stage, pathological type, depth of muscular invasion, lymphovascular space invasion, and molecular subtype were significantly different (all <0.01). There were no significant differences in 5-year RFS (=0.184, =0.113) and DSS (=0.549, =0.247) among patients with different CD T lymphocyte density and PD-L1 protein expression. Further analysis according to molecular subtype, the results of CD T lymphocyte density and PD-L1 protein expression showed that the 5-year RFS and DSS of B7-H4 protein-positive patients were higher than those of B7-H4 protein-negative patients with NSMP subtype, low density of CD T lymphocyte and PD-L1 protein-negative endometrial carcinoma (all <0.05), however, there was no significant difference in 5-year DSS between B7-H4 protein-positive patients and B7-H4 protein-negative patients with PD-L1 protein-negative endometrial cancer (=0.060). Multivariate analysis showed that positive expression of B7-H4 protein was an independent factor for 5-year RFS (=0.27, 95%: 0.09-0.78, =0.016) and DSS (=0.16, 95%: 0.05-0.58, =0.005) in patients with NSMP subtype endometrial carcinoma. In patients with low-density CD T lymphocytes endometrial cancer, positive expression of B7-H4 protein was an independent factor for 5-year RFS (=0.45, 95%: 0.26-0.80, =0.006), but it was not an independent factor for 5-year DSS. In patients with PD-L1 protein-negative endometrial cancer, B7-H4 protein was not an independent factor for 5-year RFS. B7-H4 protein expressed highly in endometrial carcinoma tissues, and its high expression is closely related to clinicopathological features, molecular subtype of p53 mutant and NSMP, and the favorable prognosis of patients with low density of CD T lymphocyte immunophenotype endometrial carcinoma.