Taken 16 Quadrature Amplitude Modulation (16QAM) as an example, the performance of three kinds of LDPC codes over Galois fields (GF(2m)) for m=1,2,4 in 16QAM systems was analyzed. The simulation results show that 4-ary LDPC code, rather than 16-ary LDPC, shows significant advantages in both error correction performance and decoding efficiency. Then in order to further improve the performance of NB-LDPC codes in high-order modulation systems, the watermark symbols are introducted into 4-ary LDPC codes in 16QAM systems. Compared with traditional non-binary LDPC decoding, the average iteration number of watermark scheme presents a quite obvious decrease, it means that the decoding efficiency can be greatly improved by using watermark scheme. Moreover a further Net Code Gain (NCG) improvement of about 0.1 dB can be attained when bit error ratio BER=10-5.