Fibroblast activation protein (FAP) is a promising diagnostic and therapeutic target in various solid tumors. This study aimed to assess the diagnostic efficiency of Ga-labeled FAP inhibitor (FAPI)-04 PET/CT for detecting lymph node metastasis in non-small cell lung cancer (NSCLC) and to investigate the correlation between tumor Ga-FAPI-04 uptake and FAP expression. We retrospectively enrolled 136 participants with suspected or biopsy-confirmed NSCLC who underwent Ga-FAPI-04 PET/CT for initial staging. The diagnostic performance of Ga-FAPI-04 for the detection of NSCLC was evaluated. The final histopathology or typical imaging features were used as the reference standard. The SUV and SUV, Ga-FAPI-avid tumor volume (FTV), and total lesion FAP expression (TLF) were measured and calculated. FAP immunostaining of tissue specimens was performed. The correlation between Ga-FAPI-04 uptake and FAP expression was assessed using the Spearman correlation coefficient. Ninety-one participants (median age, 65 y [interquartile range, 58-70 y]; 69 men) with NSCLC were finally analyzed. In lesion-based analysis, the diagnostic sensitivity and positive predictive value of Ga-FAPI-04 PET/CT for detection of the primary tumor were 96.70% (88/91) and 100% (88/88), respectively. In station-based analysis, the diagnostic sensitivity, specificity, and accuracy for the detection of lymph node metastasis were 72.00% (18/25), 93.10% (108/116), and 89.36% (126/141), respectively. Tumor Ga-FAPI-04 uptake (SUV, SUV, FTV, and TLF) correlated positively with FAP expression ( = 0.470, 0.477, 0.582, and 0.608, respectively; all ≤ 0.001). The volume parameters FTV and TLF correlated strongly with FAP expression in 31 surgical specimens ( = 0.700 and 0.770, respectively; both < 0.001). Ga-FAPI-04 PET/CT had excellent diagnostic efficiency for detecting lymph node metastasis, and Ga-FAPI-04 uptake showed a close association with FAP expression in participants with NSCLC.