The progressive failure of expansive soil slope at shallow depth occurs frequently; but the traditional limit equilibrium theory can not reflect properly particularities of shallow instability of expansive soil slope. Based on a series of large static physical model tests of the compacted expansive soil slope, this paper firstly simulated the moisture field distribution by the unsaturated model of two-phase flow of FLAC; and then, by referencing the temperature expansion theory, a back analysis of the moisture swelling coefficient of expansive soil is conducted according to the indoor expansion tests with or without vertical loading. Finally, a multifield coupling numerical analysis method with moisture field, swelling deformation field and stress field is proposed. Combining the physical model tests with multifield coupling numerical analysis, the expansion failure mechanism for the shallow instability of expansive soil slope is expounded.