Abstract This paper presents a semi‐analytical solution of undrained cylindrical cavity expansion/contraction problem using a two‐surface plasticity model for natural stiff clays from the view point of poromechanics. The present solution reduces the classical boundary problem into a group of seven first‐order ordinary differential equations via an auxiliary independent variable ξ under undrained conditions. It strictly follows the premise of constant soil mass (CSM). Meanwhile, the solution for the constant soil volume (CSV) problem is used for comparison. It shows that CSM assumption is of paramount importance in developing solutions under high stress condition, while CSV induces non‐negligible errors in effective stress and pore pressure distributions. Based on the premise of CSM, extensive parametric studies on cavity expansion/contraction are undertaken to investigate the effects of the model key parameters on the stress components distribution, excess pore pressure distribution, and cavity expansion curves. The present solution could be useful while dealing with geotechnical and petroleum engineering problems involving stiff clays or soft claystones. The application in Praclay Gallery excavation in natural Boom clay well demonstrates the validity of the present solution.