The internal energy of working fluid in Twin-Rotor piston engine is transmitted into mechanical energy by differential velocity movement between the couple of rotors coaxial installed. The blade piston, a part of the power transmitting mechanism, is the most important component working in the worst condition in the engine. It withstands heavy mechanical and thermal loads as working at a high rotating speed, that is to say it undertaking the explosive force straight and transmitting the power to power transmitting mechanism at the first time. So the section shape of blade piston on the rotor affects power output of the engine directly. In order to select the section shape of blade piston properly, improve output capacity and reduce the affect of deformation, models of several typical shapes of the blade piston is set up. Based on the comparison of aspect ratio, analysis and size optimization are performed. The results show that the rectangular cross-section blade piston is more efficient in power transmission with small deformation and larger torque as long as the values of aspect ratio are in a reasonable range.