Abstract: A hybrid algorithm suitable for high-order QAM signals is presented. Equalization of the in-phase and quadrature components of constellation symbols is implemented respectively at the first mode. Equalizer's outputs are decided by square ring decision region. The proposed algorithm switches over to Decision-Directed equalization scheme smoothly once the error level is reasonably low. Simulation tests are conducted using underwater acoustic channel model. The results prove that the proposed algorithm has the advantages of robustness, low residual mean square error, cold start and restart ability, and can greatly reduce the error rate and improve the robustness of high-speed underwater acoustic communication.