In this paper, motion formulae of two degree freedom seven-bar mechanism are derived a mathematical model is established for optimal design of the seven-bar leg mechanism of mobile robot. The elliptic trajectory curve with the long half axis of 100 mm and the short half axis of 40 mm is selected as the relative ideal trajectory curve of robot foot endpoint. The two degree freedom seven-bar robot leg mechanism is optimized by using the genetic algorithm. The foot endpoint actual trajectory curve of the optimized seven-bar leg mechanism of robot almost coincides with the ideal elliptic trajectory curve. The maximum and minimum x coordinates of the foot endpoint are respectively 100.035 mm and 100.036 mm, and the maximum and minimum y coordinates are respectively 40.005 mm and 40.009 mm. The maximum relative error with the long axis and short axis radius of the ideal ellipse trajectory is only 0.036%. Applying the optimized two degree freedom seven-bar robot leg mechanism, the hexapod simulation robot is designed. The motion simulation of the optimized seven-bar robot leg mechanism and the hexapod simulation robot are carried out by UG software, and the simulation result are satisfactory.