Abstract: To ravel the application range and some noticeable problems of the earth’s equivalent radius method during using this model to simply calculate the atmospheric refraction of radio waves, the mapping relationships of the variables between the earth’s equivalent radius model and the original earth model were derived from the definition of the earth’s equivalent radius method. The calculation results show that the height, elevation and range of ray is equal in the two models, geocentric angle keeps a rate about 3/4 in the equivalent procedure. The height, elevation and range of a radar ray in the earth’s equivalent radius model can be directly used to replace the relevant parameters in the original earth model. The Geocentric angle should be avoided as possible. Two samples were given to discuss the cases when the geocentric angle must be used, as well as the results and error analyses.