On the basis of the limit equilibrium theory, we took a sand slope as research object and deduced the expressions of the slope safety factor with quasi-static method and pseudo-dynamic method when steady seepage and earthquake were present. The results of computer programs agree well with those of exiting methods, which verifies the validity of the two analytic methods. Parameter analysis shows that there is a linear relationship between hydraulic gradient and safety factor, and the higher the water level, the easier the slope is to lose stability. When the seepage direction is downward, the slope stability increases with the increase of hydraulic gradient, and when the seepage direction is upward, the change rule is opposite. The slope stability increases significantly with the increase of horizontal seismic acceleration factor, and the effect of vertical seismic acceleration factor on the slope stability is relatively small, which can be ignored in simplified calculation. Generally, the analytic formula of pseudo-static method is simple and practical, and the formula of the pseudo-dynamic method can more comprehensively investigate the characteristics of the slope stability changing with time.