Abstract: [ Objective]: To investigate the effect of lower body mass index before operation on functional recovery after total knee arthroplasty. [Method]: A retrospective analysis was performed on 111 patients who underwent total knee replacement in our hospital from January 2015 to January 2017, including 81 cases in normal BMI group (18.5 kg/m2≤BMI<25 kg/m2) and 30 cases in low BMI group (BMI<18.5kg/m2). The hospitalization time, intraoperative blood loss, incidence of postoperative thrombosis, perioperative blood transfusion rate, postoperative rescue rate, early postoperative VAS score, Rom recovery situation, KSS score and incidence of complications were compared between the two groups. [Result]: The incidence of wound complications, perioperative blood transfusion rate, hospitalization time and early postoperative VAS score in low BMI group were higher than those in normal BMI group, and the difference were statistically significant (P<0.05). There was no statistical difference in intraoperative blood loss, Rom recovery situation, KSS score or incidence of complications between the two groups (P>0.05). [Conclusion]: Patients with lower BMI can achieve the same clinical effect as patients with normal weight do in the case of performing total knee replacement, but early postoperative pain symptoms were more obvious , along with higher rate of the blood transfusion and longer hospitalization time.