Abstract: This paper analyzes the mechanism of physical layer secrecy coding and calculates the secrecy rate for the Gaussian wiretap channel under the security gap constraint. Furthermore, a classified normalized decoding algorithm with two-dimensional information correction based on Belief Propagation (BP) algorithm and its improvements for the short and medium block length Low Density Parity Check (LDPC) based secrecy codes is presented. The algorithm first utilizes classification according to the absolute values of incoming messages in check nodes. Then it uses 2-dimensional normalization to correct the minimum and sub-minimum values. The 2-dimensional normalized factors can be calculated respectively by using probability and statistic theory in the initialization step. Simulation results show that the proposed algorithm achieves better performance than BP algorithm and normalized BP-based algorithm at the high SNR, but the bit error radio gets close to 0.5 rapidly at the low SNR. It can reduce security gap of LDPC-based secrecy codes with different secrecy rates efficiently.