Abstract: The cylindrical cavity expansion theory provides the theoretical basis for analysis of pile installation and pressuremeter tests. However, most of the studies are conducted on cavity expansion in saturated soil, and are unsuitable for solving the engineering problem in unsaturated soil. Therefore, based on the critical state soil model, the cylindrical cavity expansion in unsaturated soil under undrained conditions is solved. The stress-strain relationship is derived considering suction effects. Meanwhile, a constitutive equation for the relationship between the suction and the void volume is introduced. Combing the radial equilibrium equation, the formulation of the problem is reduced to solving a system of the first-order ordinary differential equations. Introducing an auxiliary variable, all of the differential equations are transferred to the Lagrangian description and solved by taking the values at the elasto-plastic boundary as the initial value. The results show that the soil stresses and the cavity expansion pressure increase as the suction increases, which can be regarded as the suction-stiffening effects. For the soil with low initial ratio volume, the partial volume and the suction in the plastic zone firstly increase, and then decrease, which implies that the shear dilatancy occurs during the cavity expansion. The results have a potential application in the related projects owing to the simple constitutive relation and available parameters.