A method is proposed based on Cramér-Rao Low Bound for multi-sensor assignment of land-based air defense weapon systems.The Cramér-Rao low bound is introduced into the multi-sensor assignment model according to the characteristics of tracking.Therefore it needs not to choose target tracking algorithm during assignment and the model is made more close to actual situation by detailing the constraints.Then the Discrete Particle Swarm Optimization (DPSO) algorithm is used for calculation.The searching strategy the inertia weight and learning factor are improved to increase the accuracy and timeliness.Finally the application example is given and the results show the feasibility of this method and the validity of the improved DPSO algorithm.