Abstract: Human body modeling is an important research work in the fields of biomedicine andcomputer engineering. On the basis of NURBS FFD （free-form deformation）, a motion modelingmethod is discussed to simulate the bend of human leg and corresponding de formations of muscles. Generally, FFD uses a trivariate tensor product spline to transmit deformations, and it is feasible to choose only two-order B-spline basis function in this case. According to the anatomic structure of joints, the simulation formulas are presented, and they coincide with the motion characteristics of knee joint and muscles. In the final experiment, a proper selected lattice space and these formulas yield a good visual effect of simulation. Compared with the method of curved patches, this method is simple and efficient. The formulas can be easily applied to simulate the motion of other joints.