Under the background of navel air-defense of key positions, analysis is made to the key influence factors of target assignment problem under Dynamic Fire Access (DFA), and the constraint conditions on time, space and resource are presented. Then the methods for calculating the ready-to-launch zone as well as the deadline of integrated firepower unit are given. The assignment problem is considered as the optimal matching among launch node, guide node and target. The Anytime algorithm based on Discrete Particle Swarm Optimization (DPSO) is designed for solving the assignment problem. Simulation result shows that the algorithm can obtain the optimal assignment scheme rapidly under given constraints of time, space and resources.