Offboard active decoys (OADs) can effectively jam monopulse radars. However, for missiles approaching from a particular direction and distance, the OAD should be placed at a specific location, posing high requirements for timing and deployment. To improve the response speed and jamming effect, a cluster of OADs based on an unmanned surface vehicle (USV) is proposed. The formation of the cluster determines the effectiveness of jamming. First, based on the mechanism of OAD jamming, critical conditions are identified, and a method for assessing the jamming effect is proposed. Then, for the optimization of the cluster formation, a mathematical model is built, and a multi-tribe adaptive particle swarm optimization algorithm based on mutation strategy and Metropolis criterion (3M-APSO) is designed. Finally, the formation optimization problem is solved and analyzed using the 3M-APSO algorithm under specific scenarios. The results show that the improved algorithm has a faster convergence rate and superior performance as compared to the standard Adaptive-PSO algorithm. Compared with a single OAD, the optimal formation of USV-OAD cluster effectively fills the blind area and maximizes the use of jamming resources.