A frequency mixing method with photon counting level weak local oscillator (LO) and echo signal is put forward to decrease the shot-noise from the strong LO. The receiver is multi-pixel photon counter(MPPC), the central frequency of the mixing signal is 1 MHz, and the optical path differences are 30, 45, 60 m respectively. The experimental analysis shows that power spectral density (PSD) of every data segment has a great number of noise no matter what the distances is. Accordingly, the method of PSD averaging is used to extract the signal from the deep noise, then, matched filtering method is also introduced to enhance the signal-to-noise ratio (SNR). The PSD averaging and matched filtering method are adopted for the frequency mixing signal at the distances of 45 m and 60 m from the signal receiving system, as a result the SNRs are improved by 5.90 and 2.73 times compared with the original PSD signal, and the higher the initial SNR is, the more prominent the improvement of SNR is.