Abstract: G.9960 is a standard for next-generation home networking being developed by the International Telecommunication Union′s Standardization Sector (ITU-T). The standard adopts Orthogonal Frequency Division Multiplexing (OFDM) and Quadrature Amplitude Modulation (QAM), to achieve high through out and reliable data transmission. In this paper, we specify the algorithm of high order QAM and soft-decision decoder. Even-order modulation uses square-shape constellation map; the binary sequence is recognized as Gray code, and is mapped to complex number. Odd-order modulation use scross-shape constellation map; the binary sequence is recognized as Gray code, firstly mapped into complex number, and then rotated according to some formulation. Based on Bahl′s maximum posterior probability decoding algorithm for incoming each symbol, logarithm likelihood ratio (LLR) algorithm is deduced. As LLR algorithm refers to logarithmic operation, maximum value is used to substitute logarithmic and exponential operation to simplify the computation. Simulation results show that, when signal noise ratio (SNR) is 8dB, the bit error ratio (BER) of 2048QAM-IDPC system can reach to 3×10^-5, and the BER of 2048QAM-LDPC system can reach to 5×10^-4, in AWGN channels. Simplified soft-decision demodulation algorithm has low calculation amount and BER. In hence, it is a well-behaved algorithm.