Objective To study the changes of angle of motion after joint replacement in patients with ankylosing spondylitis with bony fusion of bilateral hip joint and knee joint.Methods Four patients with ankylosing spondylitis involving both hips and knees, resulting in joint fusion, were treated with bilateral total hip arthroplasty and total knee arthroplasty. The range of motion of each joint was observed before operation, 1 week and half a year after the second operation. Results One week after the second operation, the flexion range of motion of hip joint was (91.25±5.82) °and that of knee joint was (99.38±5.63) °. Six months after operation, the flexion range of motion of hip joint was (62.50±20.18) °, and that of knee joint was (56.88±25.06) °. Up to half a year of follow-up, 2 patients could walk on crutches, and the other patient could only walk with the assistance of a special suspension，the last patient was bedridden due to complications. The range of motion of each joint of the three patients was significantly lower than that of one week after operation. Conclusion After joint replacement in patients with ankylosing spondylitis. Although the initial effect is good. However, there is still a lot of loss of range of motion in the later stage.We consider that would closely related to the soft tissue conditions and postoperative functional exercise.