We propose a modulation assisted preprocessing scheme to reduce the computational complexity of non-binary LDPC decoding. With the proposed scheme, the received signal is directly mapped onto a truncated symbol vector through table lookup. The mapping table can be built off-line based on the characteristics of the constellation structure. The receiver needs only to calculate and, if necessary, rank the reliability of the symbols in the truncated vectors, which avoids unnecessary reliability calculation and ranking during on-line operation. The complexity analysis shows that the proposed preprocessing scheme leads to much lower computational complexity, in terms of ranking and addition operations, while offering the same error rate performance as conventional EMS preprocessing scheme.