The cost function of cooperative multi-task assignment for multiple UAVs is studied.The two main objective functions for the multi-task assignment are minimum cumulative distance travelled by all the UAVs to perform the required tasks and minimum time for the UAV team to accomplish all the required tasks.Both of them have their own limits, and there are conflicts between them.On this basis, a weighted sum method is employed, and a new cost function that combining the above two performance criterions is established.By choosing different weight values, the intention of the operator can be reflected.The discrete particle swarm optimization algorithm is applied to solve the cooperative multi-task assignment problem.Simulation results demonstrated the feasibility of the proposed cost function and algorithm.