Miniature Air Launched Decoy (MALD) is an electronic warfare technique for inducing an angular deception in a monopulse radar by recreating glint angular error. MALD flies cooperatively with the true target, forms unresolved group targets within the radar beam, and destroys the detection, tracking and parameter estimation of monopulse radar for the true target. In this paper, a typical scenario for one target and one decoy was discussed, and the measurement model of target and decoy based on the actual non-ideal sampling conditions was established. The joint multi-targets probability density was adopted to dynamically describe the number and state of the targets within the radar beam. Based on the original observation without threshold decision, a joint detection and tracking algorithm for unresolved target and decoy was proposed under the Bayesian framework, and the judgment of existence of jamming and the target state estimation were deduced. Simulation results showed that the proposed method enabled quick detection of the appearance of MALD and estimated the state of target with minimal delay and high precision. Stable tracking of the true target was achieved under severe jamming conditions.