Abstract: Three-dimensional pantograph mechanism was used as walking mechanisms because of the motionaldecoupling at its principal planes.There are many favourable respects when cylindrical three-dimensionalpantograph mechanism is used as a walking mechanism.In this paper,the motional constrained conditionsof the mechanism used as walking mechanism are studied.The mechanism was optimally designed on theconstrained conditions.The constrained optimal mechanism with the largest motional space was obtained.The optimal result was used to an actual prototype.