The TD/CDMA TDD system with unbalanced timeslot allocation between uplink and downlink is an appropriate method to distribute asymmetric traffic effectively in the third generation mobile communication systems. One of the benefits of the TD/CDMA TDD system is an efficient statistical multiplexing on a large common pool of available resources, i.e., slots defined in time and code domain. However, in order to control the time varying asymmetry of traffic between uplink and downlink for the maximum throughput, a dynamic boundary movement is also required to get an optimum timeslot allocation. In this paper, a new optimal boundary movement algorithm for TDD operation is proposed, in which the downlink portion of the slots is maximized according to the intensity of access attempts by mobiles. The proposed algorithm has shown much better performance than the conventional TDD boundary movement algorithms in terms of the mean access delay of data traffic and the blocking probability of voice traffic.