The expansive character of expansive soil slope is a very difficult problem. Now the modus operandi is that a press is set on soils; but the reasonable explanation in theory is not given; and the amount of the press is obtained only according to experience. An expansive model of expansive soils(ESEM) is put forward; and the method that the pressure is confirmed with back analysis according to displacement is given. And it is pointed out that the two ways are identical that the press and distortion are set on the slope soils by formula deduction. The analysis of rainfall is taken into account; and the finite element program is given based on the Drucker-Prager model. The back analysis is set up with 2-norm. The expansive parameter is calculated by the Newton’s iteration. The connection between the 2-norm and the expansive parameter is a curve that is single-valued continuous with a single concave point. At last

the expansive parameter and the expansive press formula of the expansive soil slope are given.