For M-ary spread spectrum communication systems, more orthogonal sequences will ensure higher data rate. For a given sequence length, it is well known that the number of orthogonal sequences is not larger than sequence length. As a result, designing more sequences with low in-phase crosscorrelation is an effective method to solve data rate problem. In this paper, an expansion method of orthogonal sequence set based on computer search is proposed. By searching permutation sequences and calculating their in-phase cross-correlation function values, the expanded orthogonal sequence set may include multiple orthogonal sequence subsets. Any two sequences in every subset are orthogonal to each other, that is, their inphase cross-correlation is equal to zero. In addition, in-phase cross-correlation between sequences from different subsets is low, which will effectively improve data rate of M-ary spread spectrum communication systems when the expanded orthogonal sequence set is used.