Abstract: A robust frequency estimation scheme for acoustic underwater Orthogonal Frequency-Division Multiplexing (OFDM) system was proposed.The communication frame structure consists of a training frame and multiple data frames.The frequency offset is estimated by using the training frame,known as Rough Frequency Offset Estimation (RFOE),to overcome larger Doppler shift,while time varying and smaller frequency offsets are estimated by using every data frame,known as Accurate Frequency Offset Estimation(AFOE).Both RFOE and AFOE are calculated with the Cyclic Prefix(CP) segment which is not contaminated by multipath in the time domain.The underwater experiments achieved 133km·kbps at zero symbol error.Trial results of these two algorithms are compared.These results indicate that the algorithm is effective in time-varying and multipath underwater channels.