This paper investigates the LDPC codes decoding algorithm over Additive White Gaussian Noise (AWGN) channels, analyzes the reason that incidental error appears in decoding process, and points out the reason why the incidental error appears is that there are loops in the parity-check matrix. Then a software method, which restricts incidental error, is proposed for decoding LDPC codes. In using this method the incidental error caused by loop's influence can be effectively restricted only by properly controlling the decoding program of LDPC. By this method, decoding performance of LDPC codes and decoding program stabilization are improved greatly, and the application of LDPC codes can enter more fields.