In this paper, a new fast sub-carrier and bit allocation algorithm in multi-user OFDM systems is proposed. Under the BER and each user's rate requests, according to the information of channel state of all users, first the sub-carriers are pre-allocated, then, bit allocation is performed to every sub-carrier.Based on the above,After this, we exchange the pre-allocated subcarriers using the method intruduced in this paper and optimize iterativly to achieve the final allocate to realize the minimize the transmiting power. At the same time, this paper analyses the tradition static subcarrier-allocation method and several adaptive subcarrier and bit allocation method. According to comparing the method of different algorithms, compare the merit and defect of them. The simulation results show that the proposed algorithm performances better than the tradition static subcarrier-allocation method and convergences quickly, the performance being close to the Lagrange algorithm, while the complexity is low.