The invention discloses an underwater communication method based on a long-delay multi-path condition. The method comprises the following steps: step 1) performing frequency-spreading processing on aninformation sequence needing to send to obtain N frequency-spreading symbols containing U bit information at a signal transmitting end, serially connecting to form a data block, interleaving the datablock to obtain an information interleaving group; serially connecting P information interleaving groups in order to obtain information data after iterating numbers of P; selecting the sequence unrelated to all frequency-spreading symbols as a synchronization sequence, performing orthogonal modulation on the synchronization sequence and the information data, and then transmitting the an underwater sound channel; and step 2), at a signal receiving end, orderly intercepting the information interleaving group data corresponding to P data blocks after processing a receiving signal, recovering U bit information from each information interleaving group data by utilizing the synchronization sequence, and taking PU recovered bit information as the receiving information. Through the method disclosed by the invention, the performance of the conventional frequency-spreading algorithm under the long-delay multi-path channel is improved.