A scheme of Margin Adaptive(MA)-based improved subcarrier and bit allocation was proposed for resource allocation in multiuser Orthogonal Frequency Division Multiplexing(OFDM) system. In order to decrease the transmit power of users and guarantee the fairness principle, subcarrier exchange for user Max-Min transmission rate was adopted as the principle, besides employing proportional fairness principle to allocate subcarriers for each user. Then, the bit was rounded by the greedy algorithm with the channel state information judgment, to obtain the minimized transmission power of the system. The analysis and simulation results show that the computational complexity of the proposed improved suboptimal algorithm is a little higher than that of the traditional step-by-step algorithm, but when compared to the optimal algorithm, the proposed algorithm is of almost the same performance and a far lower computational complexity.