Abstract: The double threhold M-N detector is used to detect the low-flying target in the multipath environment.By analyzing the performance of the M-N detector working in different environments, the optimal M values for various M-N detectors in different environments and false alarm are acquired which makes radar have the best detection performance.First,the multipath scattering model for the radar is modeled according to the characteristic of the environment radar faced when detecting the low-flying target.On the basis of the modeled reflected echo,the probability density function of signal noise ratio(SNR) and the detection probability for the radar under multipath environment are derived.Finally the detection probability curves under different circumstances are simulated and the optimal M values are given under various circumstances.