Objective To investigate the relationship of bone mineral density(BMD) with age in ankylosing spondylitis(AS) patients, which might benefit prevention and treatment of low BMD or osteoporosis (OP) in AS. Methods Fifty AS outpatient (modified New York criteria) were selected and divided into two groups by age, ≤40 years and ＞40 years. BMD were measured by dual-energy X-ray absorptiometry, in lumbar (L1-L4), femoral neck and hip. T value not more than -1.0 was defined as lower bone density, including osteopenia (-2.5＜T＜-1.0) and osteoporosis (T≤-2.5). Results Based on measurement of lumbar, percentage of low BMD was higher in two AS groups than normal controls, and was higher in ＞40 years AS group than in ≤40 years group(P＜0.05). T value was lower in ＞40 years AS group than in ≤40 years group (P＜0.05). While based on measurement of femoral neck and hip, the incidences of low BMD and osteoporosis were not significantly different between two AS groups(P＞0.05). Conclusions AS patients may appear osteopenia or osteoporosis at early stage. The incidence of low BMD and the risk of fracture increase with aging. Vigilance to AS complicated with osteoporosis and timely calcium supplement are recommended clinically, to improve the patients￠ quality of life.