Objective: To investigate the effect of remaining a certain degree of flexion deformities(FD) during total knee arthroplasty(TKA) on efficacy of knee osteoarthritis with severe FD. Methods: A total of 95 patients with knee osteoarthritis with severe FD who underwent primary TKA in the First Affiliated Hospital of Nanjing Medical University from October 2013 to September 2021 were retrospectively analyzed. There were 24 males and 71 females, aged 68.85±4.83 years(range from 58 to 79 years). The body mass index was 26.01±4.50 kg/m2(range from 19.70 to 39.39 kg/m2). Based on intraoperative correction of severe FD, the group was divided into an intraoperatively completely-extended group(52 cases)and an intraoperatively incompletely-extended group(43 cases). The operation time, operative blood loss, postoperative swelling degree of lower limbs and the incidence of postoperative complications were recorded, and the hospital for special surgery(HSS)score, activities of daily living(ADLs)and visual analogue scale(VAS)were recorded to compare the differences between the two groups. Results: All patients successfully completed the operation and were followed up for 35.85±1.99 months(range from 33 to 39 months). The intraoperatively incompletely-extended patients got a shorter operation time(P=0.001), less operative blood loss(P<0.001)and milder swelling of the lower limbs(the postoperative circumference of 15 cm above the patella, P<0.001; the postoperative circumference of 10 cm below the patella, P<0.001). The incompletely-extended patients had greater FD than the completely-extended patients at 3 months(P=0.016)after surgery, but gained less pain(P=0.014). No significant differences were observed in postoperative HSS and ADLs scores between the two groups of patients. At 6 months, 12 months, 24 months and 36 months postoperatively, there were no statistically significant differences in FD, HSS, ADLs and VAS between the two groups. There was no statistically significant difference in the HSS improvement between the two groups within 3-6 months, 6-12 months, 12-24 months and 24-36 months after surgery as well. The incompletely-extended patients during TKA had a lower incidence of deep vein thrombosis(P=0.048)compared with the completely-extended patients. There was no statistical difference in the incidence of medium-term and long-term complications between the two groups. Conclusion: When treating knee osteoarthritis with severe FD, remaining a certain degree of FD during TKA does not affect the recovery of knee range of motion and function, while the incidence of deep vein thrombosis is lower.