Compared with the motion of hip's vertical oscillation, the hip model is derived in horizon motion. Based on it, the leg(thigh and shank) can be modeled as two pendulums joined in series and the rotation motion models are presented respectively. Then the mathematical models for simple two-dimension planar kneed walker are obtained. This model is based on passive dynamic walking principles. It is both simple enough to study its dynamics and performance without any linearizing, and complex enough to be able to build a prosthetic dynamics model for the prosthetic controlled system.