In order to solve the contradiction between lightweight and high reliability of electric aircraft electric propulsion system, a reliability allocation method based on weight optimization is proposed. For the first time, the analytical expression of weight and reliability is introduced into the electric propulsion system design model as constraints, and a flexible "weight-reliability" optimization method is proposed, considering the uncertainty of the operating conditions of electric aircraft. design a lightweight design scheme under typical flight profile and operation scene. Taking the "Spirit of Innovation" electric aircraft as an example, the results show that under the condition of ensuring the reliability of the system, this method can reduce the total weight of the power system of the electric aircraft by 3.5%. And the applicability of the proposed method is reflected by designing different scenarios.