Abstract: In the paper, power-spectrum time series of the extremely low frequency electromagnetic field in different item recorded by Hongshan standard seismic station is calculated and studied using diagraph analysis. The result shows that, 1) all power-spectrum densities in each frequency and observation item have sine shaped seasonal distribution characteristics, mean that, datum are dispersed in summer and dense in winter. The variation in summer is 1 to 10 times as much as in winter.2) power-spectrum densities in each frequency and observation item have no trend change for years, and its interannual variability is 1~3 orders of magnitude less than its annual variability.