In view of the wide demand for precision positioning in the ocean and the high cost of traditional precision positioning services, it was proposed to use BeiDou short-message equipment to transmit navigation observation data, combined with real-time data products, to achieve low-cost centimeter-level precise relative positioning in the ocean. In order to overcome the shortcomings of low transmission frequency of short messages, for the first time, the two technologies of fusion spatial relative positioning and precise point positioning were used to encrypt coordinates outside the minute interval, avoiding the accumulation of errors in the encrypted coordinates of the traditional time-relative method. Simulation experiments were carried out based on static and dynamic ocean observation data. Results show that the new method can achieve centimeter-level positioning in both horizontal and vertical directions, and the results show that the new method is not sensitive to the length of time interval.