Seepage flow is a negative factor to slope stability under surface water drawdown conditions, while the increase of the effective stress and strength induced by the water drawdown is positive to slope stability. Both of the negative and the positive factors should be considered in stability analysis of unsaturated soil slopes and both of them are influenced by soil permeability coefficient, range and speed of water drawdown. In this paper,finite element analysis was conducted and it showed that the slope safety factor initially decreased, and then increased in the process of water drawdown. The most dangerous status appeared in the early stage of water drawdown, and its appearance instant and the corresponding safety factor value were influenced by water drawdown speed, especially for the slopes with low permeability. A slope with lower permeability rendered a smaller minimum-value for the safety factor, and needed a longer time to recover its initial value. Sharp decrease of safety factor could be avoided by controlling water drawdown speed in a reasonable range according to different soil permeability coefficients.