Research conclusions:In this paper，a new measurement datum is presented． That is to take the WGS84 as the datum ellipsoid to firstly changes the semi － major axis and eccentricity of the datum ellipsoid and then the three translation parameters of ellipsoid centre and three rotation angles of axis can be obtained by using the deflections of the vertical component to build an optimal fitting surface of local ellipsoid with the average elevations of the construction area; At last，the independent precise measurement datum is established with the appropriate map projection method．This method is not only available for the precise measurement for the high － speed railway，but also it can provide the theory basis for the CPIII 3D adjustment．