To measure the trajectory of multi-fingers during a gripping process, a method detecting the trajectory of fingers during gripping and calculating the relationship among joint angles is proposed. Some labels are attached on fingers, then the total gripping process is captured using video cameras. The joint angles are obtained and the relationship between finger joint angles are established. Afterwards, fingers' motion trajectory is simulated according to the function. The results show that the proposed method can generate joint trajectories, which would be helpful to analyze and design humanoid robot hands. It will provide the design basis for the size design and mechanical design while designing humanoid manipulator.