In order to research the decoding performance of watermark auxiliary low density parity check scheme in high-speed coherent optical communication system ,this paper presents a forward error correction scheme of watermark symbols auxiliary low density parity check code used in the coherent communication syste m. Compared with the traditional low density parity check code,the new scheme ca n greatly improve the noise immunity of coherent receiver.After running o n the 100Gbit/s 16QAM coherent optical simulation system,the results show that a n improvement of 0.45dB signal noise ratio (SNR) gain has been realized by the proposed forward error correction scheme compared with traditional low density parity check code ,at post-forward error correction bit error rate (BER) of 10－9.Also,to obtain the same BER,the number of iterations u sed in watermark system auxiliary LDPC decoding decreases.