Analysing the mechanism of nonlinear variations in GPS stations coordinates time series is an effective way to improve the coordinates accuracy of GPS stations.The constellation design characteristic of GPS may cause the periodic variations of GPS stations coordinates and affect the precision of GPS stations coordinates.In order to analyze the systematic error of the GPS,the spectrum analysis was applied to detect the periodic terms of the GPS and VLBI vertical coordinates time series in the selected 6 amalgamated stations,the amplitude and initial phase of periodic terms were extracted.