Objective To investigate the effect of body mass index (BMI) on the short-term effectiveness of high tibial osteotomy (HTO) in the treatment of varus knee arthritis. Methods The clinical data of 84 patients (84 knees) with varus knee arthritis treated with HTO between May 2016 and August 2020 were retrospectively analyzed. According to BMI, the patients were divided into normal group (32 patients in group A, BMI<25 kg/m2), overweight group (27 patients in group B, BMI>30 kg/m2), and obese group (25 patients in group C, BMI>30 kg/m2). The BMI of groups A, B, and C were (23.35±0.89), (26.65±1.03), and (32.05±1.47) kg/m2, respectively. There was no significant difference (P>0.05) in gender, age, surgical side, disease duration, and preoperative Hospital for Special Surgery (HSS) score, visual analogue scale (VAS) score, knee range of motion, and hip-knee-ankle angle (HKA) between groups. The operation time, intraoperative dominant blood loss, and the decrease of hemoglobin on the 3rd day after operation were recorded and compared between groups. The improvement of knee joint function and pain status were evaluated by knee joint HSS score, knee range of motion, and VAS score before and after operation, and measuring the HKA of patients on X-ray film. During the follow-up, the X-ray films of the knee joint were reexamined to observe the position of the internal fixator and the healing of osteotomy. Results All patients completed the operation successfully and were followed up 8-40 months (mean, 19.3 months). There was no significant difference in follow-up time, operation time, intraoperative dominant blood loss, and the decrease of hemoglobin on the 3rd day after operation between groups (P>0.05). No operative complications such as severe vascular or nerve injury occurred. After operation, deep venous thrombosis of lower extremities occurred in 1 case in groups A and B respectively, and fat liquefaction of surgical incision occurred in 2 cases in group C. There was no significant difference in the incidence of perioperative complications between groups (3.1% vs. 3.7% vs. 8.0%) (P=0.689). During the follow-up, there was no bone nonunion, plate fracture or loosening. At last follow-up, HSS score, VAS score, knee range of motion, and HKA significantly improved in the 3 groups when compared with those before operation (P<0.05), but there was no significant difference in the differences of the above indexes between groups before and after operation (P>0.05). Conclusion BMI does not affect the short-term effectiveness of HTO in the treatment of varus knee arthritis. HTO can be selected for overweight and obese patients after standard medical treatment is ineffective.