Proceed from the detected optical field′s quantum statistics model of the Avalanche Photo Diode (APD) photoelectric detection receivers, an accurate realize method of Intensity-Modulation/Direct-Detection (IM/DD) Free Space Optical Communication (FSOC) fit Low Density Parity Check(LDPC)Codes decoding algorithm was investigated. This scheme, which was based on APD receivers′ output statistical model(Webb-Gaussian Model), derivated the accurate calculating method of the original value of the likelihood ratio in LDPC decoding arithmetic over M-PPM demodulation. A detailed decoding process was given at the same time. By taking theoretical analysis and simulations, a result has been proved that：comparing with the most frequent used asymmetric Gaussian approximation model of APD receivers, the scheme introduced in this paper could get a gain of 1-2 dB in the decoding performance while in the same SNR. This gives the theoretical support in order to improve the performance of FSOC system further more.