The relationships between radar detection probability and signal to noise ratio under two environments including free space and the multi-path are analyzed in detail. The radar detection range and the multi-path transmission factors under different reflection coefficients and different carrier frequencies are studied as well. The results show that the smaller the reflection coefficients, the closer the detection probability curves to the curves of the free space; the detection performance declines under high Signal to Noise Ratio(SNR) due to the multipath fading; in order to get the same detection probability, the SNR required in multi-path is lower by 7 dB-8 dB than that in the free space when the SNR is low; the bigger the reflection factors, the bigger the fluctuation of multi-path transmission factors, and the corresponding cracking of exposure zone is more serious.