Under the gravitational action of the moon and the sun, the earth's solid surface also has periodic fluctuations similar to the sea surface, this phenomenon is called solid tide. The cause of formation of solid tide is the same as that of sea tide, but its calculation is much complicated more than that of the latter. In this paper, the Kelvin earth model and a relatively simple method are used to calculate the earth's surface equation and fluctuation amplitude, and the related problems are discussed.