The invention relates a dynamic allocating method of uplink bandwidth of WiMAX system, comprising: firstly analyzing the relationship between collision probability, competition service stream number and competition bandwidth size of the cutoff binary index retreating algorithm, and determining the initial size of competition request bandwidth of uplink subframe of WiMAX system; then according to the allocating result, data packet arrival condition, and bandwidth request mode of the previous frame, real-timely and dynamically regulating the competition request bandwidth, and nrtPS and BE data transmission bandwidth size of the uplink subframe; in the regulating course, able to make congestion control of the request according to the number of requests which has been received by BS end but do not obtain bandwidths and the request-allowed maximum delay time, to receive or refuse a new request. And the method can effectively reduce bandwidth request collision probability, raise system transmission efficiency, improve MAC layer performance on different flow load conditions, and to a certain extent, make network congestion control, and it is an effective and practical allocating method.