During the penetration of ballistic missile targets, there are lots of baits or decoys forming group targets. Targets in the same group are so close to each other that it's difficult to track each target continuously and correctly without false data association. False data association leads to false target recognition and orbit computation, which will shorten time to intercept the ballistic missile. A data association algorithm based on optimal assignment strategy (OASDA) is proposed. First, consider the data association problem as a special case of optimal assignment problem. Then build probability assignment matrix based on normalized residuals, speed, heading, AD and other characteristic parameters. Finally solve the assignment matrix to get measure and track pairs. Simulation results show that the OASDA algorithm can significantly improve the accuracy of data association within the same target group.