Aimingat solving the problems of large amounts of computation and simplifying high complexity of M-ary low density parity check code (LDPC) decoding algorithm, a M-ary LDPC improved decoding algorithm is proposed based on the characteristics of BeiDou-3 (BD-3) B1C satellite signal. On the one hand, the proposed algorithm adopts the dimension reduction and edge complement mode to reduce the calculated amount of decoding algorithm by reducing the calculation dimension between nodes in the updating process of verification equation；On the other hand, to reduce the decoding complexity, the algorithm uses the way in which the nodes update simultaneously, which means the check node updates as the variable node updates. The simulation results show that the improved algorithm reduces the decoding computation and simplifies the decoding algorithm complexity without affecting the decoding performance in the dynamic range of receiving level of BD-3 B1C satellite signals. Compared with before optimization, the calculation amount is reduced to 25% of the original algorithm in one iteration, and 20000 times of data storaging and reading operations are saved when the calculation dimension is 32 by updating the node algorithm. Moreover, with the improvement of signal to noise ratio (SNR), the decoding computation amount and complexity can be more simplified by using smaller dimensions.