Abstract: Orthogonal frequency division multiplexing (OFDM) has the characteristics of high transmission rate and low implementation complexity, but it is sensitive to Doppler frequency shift. Accurate Doppler estimation and compensation is the key to ensuring the communication performance in the presence of the Doppler effect. To solve the problem of low accuracy and high complexity of traditional estimation methods under time-varying Doppler effects, a low complexity underwater OFDM carrier frequency offset (CFO) estimation algorithm based on conjugate phase difference information is proposed. The approximate relation between the carrier phase difference in adjacent symbols and the CFO is first derived. Then, the unknown transmission information can be eliminated based on the phase conjugation of adjacent subcarriers in the time-frequency differential OFDM system. In this way, time-varying Doppler estimation can be realized without inserting extra data auxiliary signals. Simulation and sea trial results show that the proposed algorithm has better performance than traditional methods under time-varying Doppler effects, which proves that the proposed algorithm can be effectively applied to OFDM underwater acoustic communication under time-varying Doppler effects.