OBJECTIVE: Weight is an influential factor in knee osteoarthritis (KOA). However, the effect of abnormal body weight on chitosan's efficacy in treating KOA is unclear. This study aimed to explore the differences in the effectiveness of arthroscopic surgery combined with intra-articular chitosan injection for KOA in patients with abnormal body weight. METHODS: Patients with stage II-III KOA (Kellgren-Lawrence rating, K-L) undergoing arthroscopic surgery were recruited for this clinical study from January 2020 to September 2021. Based on body mass index (BMI) and intra-articular chitosan injection, patients with KOA undergoing arthroscopic surgery (138 patients) were divided into four groups: low-weight-non-injection (Lw-N, BMI <18.5); low-weight-chitosan injection (Lw-CS, BMI <18.5); overweight-non-injection (Ow-N, BMI ≥25); overweight-chitosan injection (Ow-CS, BMI ≥25). A 2-year follow-up was conducted to evaluate various indicators, including the visual analogue scale (VAS) and the Western Ontario and McMaster Universities osteoarthritis index score (WOMAC). Statistical analyses were performed using relevant parametric or non-parametric tests. RESULTS: In total, 138 patients with KOA were included in this study. There were no significant differences in gender, age, and incidence of chronic residual pain after arthroscopy among the four groups (p > 0.05). The proportion of patients undergoing subsequent knee arthroplasty during the 2-year follow-up period was significantly higher in the Ow-CS group (20/35) than in the Lw-CS group (12/39) (p < 0.05). The K-L rating showed an overall increasing trend over time, with the K-L rating in the Ow-N and Ow-CS groups significantly higher than that in the Lw-CS group at the final follow-up (p < 0.05). VAS and WOMAC scores significantly decreased at 1 and 3 months post-arthroscopy and then increased. One month after arthroscopy, VAS was significantly lower (p < 0.05) in the intra-articular chitosan injection groups (Lw-CS and Ow-CS) compared with the non-injection groups (Lw-N and Ow-N). VAS was lower in the Ow-CS group than in the Lw-CS group (p < 0.05). There was no significant difference in WOMAC between the intra-articular chitosan injection and non-injection groups at each time point (Lw-N vs. Lw-CS, Ow-N vs. Ow-CS, p > 0.05). CONCLUSION: Arthroscopic surgery combined with intra-articular chitosan injection shows short-term positive effects in treating KOA. Intra-articular chitosan injection appears to have a greater short-term pain relief effect in obese patients.