The water content in the subgrade changes easily as the variation of the environment water, which causes the strength attenuation of the subgrade. It is harmful to the stability of subgrade slope. Strength decay pattern of silt soil in yellow river alluvial plain is studied through the laboratory test. And then the finite element software GEO-SLOPE is used to calculate coefficient of stability under different conditions. Results show strength indexes of silt soil are maximum in the optimal water content, and the change of the water content would cause strength attenuation. The stability of subgrade slope decreases as the strength degradation, while the evolution is different, which is semi-parabolic in the lower subgrade but rectilinear in the higher subgrade. Seismic load leads to stress concentrate at the slope, combined with the strength attenuation the landslide is easily to take place.