Osteoporosis could affect the life quality of post-menopausal ladies badly. The paper introduced the physiological cause of osteoporosis and the anisotropic bone modelling model with mechanostat .The model of osteoporosis was simulated and numerical calculated with finite element method . The results show that pure menopause factor results to a fall of 7%~12% bone mass. Adding the fall of 20% loads results to a fall of 25%~29% bone mass after menopause and when fall of 40% loads results to a fall of 35% .The result shows that biomechanics is primary factors leading to osteoporosis and biology factor quickens the process of osteoporosis. So the biomechanics factor is important to maintain bone mass. This result provided a new method to study osteoporosis.