Abstract: The existing expansion theory of cylindrical cavity has been able to provide theoretical basis for such as the wellbore stability evaluation in petroleum engineering and side pressure and cone penetration experiment analysis. But it is rarely applied in practical engineering problems such as unsaturated foundation pressure grouting and composite foundation treatment. Based on the theory of elastoplasticity and the principle of unsaturated soil mechanics, this paper adopts the unified strength theory to analyze the problem of cylindrical cavity expansion in unsaturated soil. Firstly, the soil around the cylindrical cavity is divided into the elastic zone and plastic zone and consider following the small strain theory in the elastic zone and the large strain theory in the plastic zone, and considering the influence of intermediate principal stress and inter-grain suction on the strength of unsaturated soil. Secondly, applying the unified strength criterion expressed by effective stress, based on basic equations such as constitutive relations, geometric equations, and momentum balance equations, combined with the corresponding boundary conditions. Finally, the analytical expressions of the stress field, strain field, displacement field and limit reaming pressure in the surrounding elastic-plastic region when the cylindrical cavity expands under different drainage conditions are obtained. Through numerical examples and parameter analysis, while degenerating verification with the existing expansion theory of cylindrical cavity in saturated and unsaturated soils, the influence laws of suction, dilatancy parameters, intermediate principal stress effect parameters and initial radial effective stress on the stress field, strain field and displacement field in the elasto-plastic region are analyzed. So as to verify the correctness and effectiveness of the theory in this article, in order to provide a reasonable theoretical basis for the later practical engineering problems.