Engineering geological characteristics of Xigeda strata in Sichuan Province was analyzed. Xigeda strata has multiple chemical composition that contains a certain amount of soluble salt, and tends to bring about chemical reaction and forms secondary minerals when submerged in ground water or surface water. This may reduce the strength of Xigeda strata and cause slope instability. Headward erosion is a kind of river erosion, and if the flow of the lateral erosion and gravity act on the slope at the same time, it would accelerate the instability velocity of slope. The main factors affecting headward erosion were analyzed using mechanical methods of soil mechanics, hydraulics etc. A generalized physical model of Xigeda strata slope was established on the basis of the unsaturated soil calculation. In order to make the model more accurate and reliable, the model took river lateral scouring and hydrodynamic pressure into consideration. Since the present unsaturated soil shear strength calculation was not unified, the stability calculation formulas of unsaturated soil slope was deduced and optimized. The stability of Xigeda strata slope was analyzed based on the headward erosion model, and the calculated stability coefficient of slope was 1.061, but slope stability coefficient under the same conditions was 1.212 using classical Bishop method. Because the coefficient calculated by the optimization calculation formula was lower.