This paper proposes a joint optimization algorithm of stealthy trajectory planning and task allocation in unmanned aerial vehicle (UAV) cluster under multi-threat environment. Firstly, a model for the multi-threat environment is established. Then, a comprehensive cost function is designed for UAV cluster stealthy trajectory planning and task allocation. Combining the scattering characteristics of the UAV itself, taking the comprehensive cost function as the optimization objective, and the platform security, the UAV's own dynamic limitations and the task allocation scheme as the constraints, to construct a joint optimization model of stealthy trajectory planning and task allocation in UAV cluster under multi- threat environment. Based on this, an improved algorithm combining A* algorithm and genetic algorithm is employed to solve the optimization model. The simulation results validate the effectiveness and superiority of the proposed algorithm.