Abstract: The damage constitutive model is of great significance for studying the fracture and failure behavior of materials, but very few studies have been conducted to characterize damage evolution in polymeric materials quantitatively. In this study, notched round bar specimens with four different notch radii, made from high density polyethylene (HDPE) are stretched under uniaxial tension until fracture to obtain load-displacement curves and true stress-strain curves. The constitutive equations for HDPE materials under different stress states are determined through a combination of experimental testing and finite element (FE) simulation. The FE model, which can successfully regenerate the experimentally determined load-displacement curves, is then applied to establish the relationship between notch radius and stress triaxiality. A two-stage test method is proposed to quantify the variation of elastic modulus in HDPE specimens under uniaxial tension. The damage evolution equations for four types of HDPE specimens are established based on the degradation of elastic modulus. In addition, microstructure evolution in HDPE specimens under different stress states has been analyzed using interrupted tests and scan electron microscopy (SEM). The results show that the smaller the notch radius, the higher the stress triaxiality. Additionally, damage initiates earlier and develops fasters in HDPE specimens with higher stress triaxiality. From the microstructural point of view, higher stress triaxiality facilities the initiation and evolution of cavities in HDPE specimens, while suppresses the formation of fibrillar structures. A new approach for the identification of parameters in damage evolution models has been proposed base on the information of fracture strain, stress triaxiality and damage evolution equations determined from experimental testing and FE simulation. The constitutive equation and damage evolution model determined using the proposed methods are applied to simulate the deformation and fracture behavior of HDPE plate subjected to punch loading. The simulation results are in good agreement with the punch test results.