This paper utilizes the contract network protocol to achieve UAV dynamic mission assignment based on the basic biding procedure. Path planning is a very important sub-problem in UAV dynamic task assignment, therefore it is worth discussing. As a type of intelligent optimization algorithm, the ant colony algorithm is frequently used in UAV path planning because of its fine characteristics of self-organization, parallelism, positive feedback and robustness. This paper uses ant colony algorithm to replan the UAV flight path during the dynamic mission allocation according to the required length cost in the biding.