In traditional orthogonal frequency division multiplexing （OFDM） systems, the realization of adaptive modulation needs to transfer a great deal of sub-carrier modulation parameters, which may decrease the transmission efficiency. In order to solve this problem, this paper analyzes the characteristics of adaptive modulation OFDM systems with data spread, and points out that an optimal algorithm is to assign the bits and power among original data symbols averagely. From this viewpoint, an adaptive modulation algorithm orientated to users＇ requirements with a minimum total power is proposed. In this new algorithm, two different constellations are adopted at most to assign the original data symbols in order. Thus, the complexity of the algorithm is greatly reduced and the transmission of modulation parameters is obviously decreased. Simulated results indicate that the proposed algorithm is good in performance, and that it saves about 2 dB power （represented by the average signal-to-noise ratio of the sub-carrier） , as compared with the traditional adaptive power control-based OFDM system with similar complexity.