Abstract: A direct-adaptation based bidirectional turbo equalizer for underwater acoustic communications is proposed. Abandoning the channel estimation process, the direct-adaptation based turbo equalizer embedded with digital phase- locked loop is adopted to track time-varying channel. The fast self-optimized algorithm is used to adjust the step size, so a good tradeoff between the convergence speed and performance has been made. Frthermore, by minimizing the mean squared error, an optimal weighting factor is derived to exploit bidirectional diversity gain. The forward turbo equalizer is combined with the backward turbo equalizer to eliminate error propagation effect. Simulated and experimental results demonstrate that the bidirectional turbo equalizer outperforms the single directional one. Seen from the experimental results, compared to the channel estimation based equalization algorithm, the direct-adaptation based algorithm is less sensitive to the time-varying channel and has a lower bit error rate.