Compared with the qualitative analysis,the quantitative analysis of VTEC (vertical total electron content) is more conducive to the modeling,fore-casting and correcting of ionosphere delay.With Empirical Mode Decomposition (EMD),the mean nighttime VTEC are investigated qualitatively by using global grid ionospheric data from IGS (the International GPS Service For Geodynamic).The evaluation is done in this paper by comparing GIMs data with the quantitative analysis result and Klobuchar model.The final result indicates that the quantitative analysis result is reliable to describe and forecast the trend of the global mean nighttime VTEC in a solar activity cycle.