Seasonal rainfall and reservoir operation can cause the fluctuation of reservoir water level and the change of pore water pressure distribution in the slope body

which is detrimental to slope stability. Based on the theory of shear strength of unsaturated soils and pore water pressure field obtained with finite element method for the transient unsaturated seepage

an improved numerical procedure for critical slip field of slope is proposed. In this procedure

the combined effect of water level fluctuation and transient unsaturated seepage on the behaviors of slopes can be taken into account. First

the transient unsaturated seepage fields of a clay/ silt slope are calculated using SEEP/W module in Geostudio to obtain pore-water pressure fields

which are imported into the improved procedure of critical slip field of slope method to evaluate changes in the factor of safety with changing water level. In addition

the influence of fluctuation velocity of water level and matric suction on the slope stability are also analyzed emphatically; and the slope stability changing course with water level fluctuation is revealed

so as to show the feasibility and reliability of the improved procedure. Moreover

the results not only show that the slope stability changing course is affected by fluctuation velocity of water level

matric suction

and so on

but also reveal that the more reasonable slope stability changing rules and essences are obtained by considering matric suction.