An improved adaptive transmission control mechanism (IATC) for underwater acoustic sensor networks is proposed in this paper. This mechanism controls transmission delay and releases collisions adaptively according to different conditions of the wireless links periodically feed back by the receiver. Our work is succeeding in releasing collisions and reducing propagation delay. Further more, the energy is reserved by reducing repeated transmission. Simulations show IATC can amend the transmission time adaptively depending on users and can save about 50% energy when bit error rate is high.