In long-term operation process, highway subgrade is subjected to strength decrease easily，and the water environment around often causes the side slope side-slipping. In allusion to the common clay, massive indoor triaxial tests have been carried on, the relationship between the shear strength index and the water content, compactness of the compaction clay has been determined. Based on the above research, slope stability analysis software GEO-SLOPE is used, to analyze common earth fill subgrade, soft soil ground, mountainous area subgrade and earthquake area subgrade separately. The impact of subgrade strength decrease on the slope stability has been studied considering different subgrade filling height. The results indicate that no matter what kind of subgrade form, strength decrease of the subgrade causes highway slope safety coefficient to reduce, this especially obvious in the high subgrade condition. Partial jitter possibly happens when strength decrease of the subgrade achieves to above 80% of the common earth fill subgrade. The subgrade side-slipping possibly happens when strength decrease of the subgrade achieves to 50% of the soft soil subgrade. The subgrade safety coefficient decreases the most when the main performance is side-slipping jitter of the intersection between fills and digs of the mountainous area part-cut and part-fill subgrade. The subgrade jitter happens when the shear strength index decreases to 50% under the combination of strength decrease and seismic force of the earthquake area subgrade. The high subgrade and the maintenance management of the special road section should be strengthened to avoid the project disaster.