Abstract: For solving the low accuracy and low reliability of vertical datum transmission caused by complex hydrological conditions and long distance, this paper proposed a method of establishing vertical datum for long-distance offshore platforms outside the Yangtze River Estuary. Firstly, the GNSS control network was established in the water area. Subsequently, combined with the quasi-geoid refinement model, the mean sea level and sea surface topography, a method for determining 1985 national elevation was given, based on which the comprehensive method for determining the mean sea level and chart datum was given. Finally, a completed scheme for the long-distance transmission of vertical datum was formed. To verify the validity of the proposed method, this paper had determined the vertical datum of the offshore platform by the proposed method. The results showed that the mutual difference of determining the 1985 national elevation was 2 mm and the standard deviations of determining the mean sea level and the depth datum L of the platform were 3 cm and 5cm respectively.