Micro-actuator is the key device for the MEMS to perform physical functions. According to a kind of bimorph thermal actuator, this article presents an optimum design method to improve the sensitivity of MEMS actuator. Based on thermal analysis of software ANSYS, the thermal displacement and distribution of temperature field can be obtained clearly. Then a series of reasonable parameters are determined by optimum calculation. The simulation comparison analyses including thermal displacement, stress distribution and fatigue life are carried out to demonstrate that the sensitivity of the optimized structure has been improved effectively by the presented optimization method.