The research on instantaneous center of human knee joint is one of more important research areas of knee joint biomechanics research. In this paper, NDI OPTOTRAK optical motion capturing system was adopted to measure the movement information of knee joint of one adult male. A simple and efficient method about calculating instantaneous center of human knee joint was provided. To avoid experiment error caused by improper operation, Planar fitting and cubic polynomial fitting was applied in processing data. The result provide reference for assembling and designing clinical prosthetic knee.