This paper analyzes the mechanism of geogrid-broken stone pile reinforced sand-gravel cushion soft base strengthening system (GSPS) using finite element based on the substantial project of a expressway in Sichuan province. The calculation results indicate that the GSPS system can significantly improve the vertical displacement (settlement) of subgrade with the vertical displacement around pile decreasing around 15%, and the subgrade displacement tends to be more uniform and smooth. Also, the GSPS system improves the horizontal displacement of subgrade the most compared with the common broken stone-pile, sand cushion and reinforced cushion. The embankment becomes relatively light when it is low, and the main top loads are borne by geogrid-broken stone piles. With the top loads increasing, the loads shift to the soil among piles, and the pile-soil stress ratio increases with the increase of stress level. The test and analysis results show that the value range of pile-soil stress ratio is 1.88~3.40.