This paper presents a method for analysing and optimizing the design of a compliant transmission mechanism for a flapping-wing aerial vehicle. Its purpose is of minimizing the peak input torque required from a driving motor. In order to maintain the stability of flight, minimizing the peak input torque is necessary. To this purpose, first, a pseudo-rigid-body model was built and a kinematic analysis of the model was carried out. Next, the aerodynamic torque generated by flapping wings was calculated. Then, the input torque required to keep the flight of the vehicle was solved by using the principle of virtual work. The values of the primary attributes at compliant joints (i.e., the torsional stiffness of virtual spring and the initial neutral angular position) were optimized. By comparing to a full rigid-body mechanism, the compliant transmission mechanism with well-optimized parameters can reduce the peak input torque up to 66.0%.