The impact of ISRJ (Interrupted-Sampling Repeater Jamming) on radar constant false alarm rate detection and the generation principle of false target using ISRJ were analyzed, and then generation methods of false targets jamming were given. On this basis, the detection cost caused by the false alarm and false dismissal in a search period of radar was chosen as the criteria of detection performance. The detection costs of CA-CFAR, OS-CFAR, CMLD-CFAR detectors for active decoy using IRSJ were analyzed, especially the influence of key factors of ISRJ on detection costs. Simulation result shows that the detection costs will increase remarkably for the active decoy from IRSJ and it is sensitive to the jammer power and duty ratios, but have little relationship to the repeating frequency. Among the three detectors, the detection costs of radar CMLDCFAR detectors are higher than those of the OS-CFAR and the CA-CFAR.