In the background of heterogeneous UAVs performing suppression of enemy air defences(SEAD) mission, this paper establishes the models for the cooperative task assignment problem, designs the solving algorithm, and carries out the simulation analysis. Firstly, the paper employs the graph theory to transform the problem to a directed graph, and uses the Dubins Car model to stand for the UAV's kinematic model. Then, the paper takes the constraints of the subtask including searching, verifing and attacking. Furthermore, the paper adopts a distributed framework to the genetic algorithm in order to improve the operation efficiency of the algorithm. Finally, simulation results show the practicability of the proposed algorithm.