An algorithm based on Compressive Sensing(CS) technology is proposed, which is applied in the opportunistic radar system, conducting Constant False Alarm Rate(CFAR) target detection. Because of the sparseness of the target echo on the distance unit, a new CA-CFAR detector based on the compressed domain is designed by using the compression sensing sample. In this way, the target echo can be found without restoring the original signal. The algorithm can achieve target echo detection rapidly. And the theoretical analysis of the detection threshold of the CA-CFAR detector on the compressed domain is further studied. The performance of the system and the characteristic of the receiver are given. The simulation results show that the algorithm can directly detect the low SNR signal without signal reconstruction and cut the computation amount.