A hybrid square contour algorithm for 16QAM signals is proposed.By judging whether the output signals are in the judgement domain of the constellation points and switching the algorithm between the square contour algorithm (SCA) and decision-directed (DD),this algorithm can recover the phase and reduce the Intersymbol Interference (ISI).Simulation results with underwater acoustic channel verify the significant improvements of the proposed algorithm in the convergence rate and ISI as compared with the conventional SCA.The ISI of the proposed algorithm is low,thus can provide underwater acoustic channel with significant equalization ability.