Objective To investigate the effect of the morphological changes in the proximal femur on the prothesis selection in the total hip arthroplasty in the patients with ankylosing spondylitis. Methods The experimental group was composed of 13 patients (16 hips) with ankylosing spondylitis, which was treated with the total hip arthroplasty, and the control group was composed of 16 patients（19 hips）with non-ankylosing spondylitis，which was also treated with the total hip arthroplasty. In the two groups, the measurements of Singh index，canal flare index，morphological index of the cortex and cortical index were performed in the two groups. Results The results of the statistical analysis on Singh index，canal flare index, morphological index of the cortex and cortical index in the experimental group were 3.81±0.54, 2.63±0.41, 2.02±0.38 and 1.69±0.69, respectively, but 4.63±0.62, 3.03±0.27, 2.76±0.28 and 2.12±0.24, respectively in the control group. Therewas a significant difference in Singh index, canal flare index, and morphological index of the cortex between the two groups (Plt;0.05)，while there was no statistical difference in cortical index between the two groups (P＞0.05). The patientswith ankylosing spondylitis had more serious osteoporosis in their proximal femur. Conclusion Cemented femoral prosthesis should be used in the total hip replacement in patients with ankylosing spondylitis, and the revision total hip arthroplasty should be performed on patients with more serious osteoporosis.