For low-density parity-check (LDPC) codes, reducing the number of Euclidean projections, choosing a suitable scheduling strategy, and devising an improved penalty function are three effective ways to increase the alternating direction method of multipliers (ADMM) penalized decoding speed. By combining the above three ways, this paper proposes a horizontal layered scheduling ADMM penalized decoder based on the improved penalty function to speed up decoding. The experimental data indicate that the proposed decoder can obtain better performance and lower average number of iterations than the existing ADMM penalized decoders.INDEX TERMS Horizontal layered scheduling, alternating direction method of multipliers, penalized decoding, low-density parity-check codes