To evaluate the static performance and design optimally a parallel mechanical leg for the six-legged robot, a static analysis method based on the mapping of both constraint and actuation forces was proposed,and the mechanical leg was designed based on this method. Firstly, the constraint and actuation Jacobian matrixes were established based on the mapping of both constraint and actuation. Then, according to the virtual work principle, the actuation statics transmission equation was established, the performance evaluation indexes of actuation statics were designed,and the relationship curve between the performance evaluation index and structure parameters was calculated. By the same way, the transmission equation and performance evaluation index of constraint statics were obtained,and the relationship curve between the performance evaluation index and structure parameters was also calculated. Finally, based on the performance evaluation indexes both of constraint and actuation statics, the structure parameters were optimally designed by Monte Carlo method. Calculations show that when the structure parameters of a fixed platform and a movement platform are 200 mm,and 80 mm and the UPU branched chain lengths l1min,l1max are 500 mm and 900 mm, the statics comprehensive performance of the parallel mechanical leg is the best. These studies lays the theoretical foundation for further study of the six-legged robot.