Objective To explore the effect of avascular necrosis cystic degeneration on distribution of bone density. Methods Based on the bone reconstruction model of Weinans and Huiskes, bone density distribution in normal femoral head and in femoral head with avascular necrosis cystic degeneration were calculated by finite element analysis. Results (1) The medial system of lamellae, lateral system of lamellae, intertrochanteric arch and Ward's triangle were generated on the normal proximal femur under simulation. (2) If a primary cystic degeneration occurred, a secondary cystic area would appear below the primary one and its bone density would decrease dramatically with the primary cystic area increased. Cystic degeneration would also change the femoral bearing truss system. Conclusions (1) Wolff’s law on bone remodeling is consistent with modern topology optimization theory. (2) If cystic degeneration appeared due to avascular necrosis of the femoral head, it should be treated timely to prevent the secondary cystic degeneration and collapse of the femoral head.