Abstract: This paper carefully examines the definition of the relativistic geoid, firstly in this paper, and suggests to apply the equi-frequency surface and the equi-frequency geoid to geodesy in the future. The possible direct measurements of determining the geopotential on the earth's surface as well as the equi-frequency geoid are proposed. The difference between the classic geoid and the equi-frequency geoid should be considerd, which has been investigated and concluded to be around 0.5 cm. Moreover, the expression of the equi-frequency geoid is given.