Due to seawater scattering characteristics, Inter-Symbol Interference (ISI) will occur in underwater transmission, especially in poor transmission environment. In order to solve this problem, pre-equalization technology is applied to underwater optical communication system based on Direct Current Offset Orthogonal Frequency Division Multiplexing (DCO-OFDM) modulation, and a pre-equalization method based on prior information is proposed. The performance of Turbid Harbor seawater channel is compared with that of traditional equalization algorithm. The simulation results show that the ISI in underwater optical communication system can be effectively reduced, as well as the Bit Error Rate (BER). The reliability of channel estimation can be improved under the condition of poor Signal Noise Ratio (SNR). Compared with the Least Squares (LS) post-equalization, LS pre-equalization and Mini-mental State Examination (MMSE) algorithms, the BER can be reduced by ~3.56, ~3.00 and ~1.85 dB, respectively.