In on-board regeneration satellite communications, the burden of satellite transponder is light when on-board processor works without channel decoding and recoding, but LDPC performance degradation will occur because the bit posteriori probability can not estimated precisely in LDPC classic MP algorithm. This paper researches on the algorithm of estimating the posteriori probability precisely in the downlink. Firstly a satellite communication model is constructed and the kernel of posteriori probability estimation puzzles is analyzed, then an advanced posteriori probability estimation algorithm is derived by means of probability theory. The matlab simulation displays 0.7 dB gain of the enhanced algorithm when the SNR in the uplink and that in the downlink are equivalent, and the bit error probability is 10-5. In the regeneration satellite communication system without on-board decoding and recoding, the advanced algorithm can be used to improve the performance of the receiver.