An egg-shaped yield surface elastoplastic constitutive model suitable for saturated soft clay was proposed to describe the strength and deformation characteristics of saturated soft clay and overcome the defect that the direction of the plastic strain increments at the corner of the yield surface is insufficiently defined in the traditional elastoplastic model. Firstly， the ESF elastoplastic model was formulated based on the egg-shaped function （ESF）， adopting the associated flow law and employing the plastic volumetric strain as the internal variable. By introducing a closed and smooth egg-shaped yield surface， the corner problem in the traditional yield surface form was avoided. Additionally， by adjusting the value of egg-shaped parameters， the yield surface was flexibly transformed into various forms， including bullet-shaped and elliptical forms， which reflected the dependability and universality of this model. Subsequently， the undrained stress path triaxial tests were conducted on Hangzhou saturated soft clay to investigate its mechanical characteristics. The acquired data were used to analyze the stability of the model parameters under different confining pressures. Furthermore， a comprehensive analysis of the mechanical characteristics of the soft clay was performed， and the method for determining the model parameters was clarified. Finally， the finite element program using an implicit integration algorithm was developed， and its effectiveness was demonstrated through triaxial tests conducted on Hangzhou saturated soft clay and saturated kaolin clay from relevant literature under undrained conditions. The results revealed a satisfactory agreement between the simulated and experimental data， indicating that the proposed model can reasonably describe the behavior of various types of saturated soft clay under undrained conditions.