The soil resistance is a nonlinear function varying with depth, and it is related to the effect of location in slopes. The simplified model for a pile-column bridge pile and the total potential energy equation of pile-slope system were presented by considering the slope effect. The analytical solution of the critical load and calculated length were derived based on the energy method. In this respect, verification between the measurement and theoretical solution was conducted by dissimilar conditions in plain and slope. The parametric study shows: the increase of the Young's modulus of the pier column or decrease of its free length enhances the stability of the pile foundation; the appointed elastic modulus of pier column corresponds to an optimal column-height, and the best stability is achieved while the ratio is in the range of 0.3 to 0.4. Moreover, a key conclusion from observations is that the bridge pile should be located at the slope with the gradient less than 35° for safety. However, the correlation between m and α is of significance so that it needs to be further investigated.