The monitoring of sea level change is a hot issue in the field of marine environment and global climate change research. With the rapid development of GNSS theory and application, GPS-MR technology based on multipath effect has been proved by scholars to be used for tidal level monitoring. In order to further expand the application space of GPS-MR technology in the field of marine environment monitoring, this paper conducts research on offshore sea level change based on tidal level change data acquired by GNSS-MR. This paper first gives the calculation process of GPS-MR technology to obtain the daily average sea surface, and then obtains the high-precision daily average sea surface and month by using the measured data of GNSS at different time intervals and different frequencies on the shore of the Harbour Harbor in Washington, USA. The average sea surface is in good agreement with the results of the tide check station. The experimental results show that GPS-MR technology has higher precision for obtaining tidal level change and regional mean sea surface change, and lays a foundation for GPS-MR technology to supplement satellite altimetry for offshore sea level change and to carry out offshore sea tide model refinement.