Abstract: The stress relaxation normally occurs during the hydrostatic test of PE-HD due to the viscoelasticity, which leads to a change in internal pressure. This work focused on a study on the law of pressure variation during the hydrostatic test of PE-HD on the basis of the relaxation modulus equation of PE-HD, the long-term mechanical model of pipes and the state equation of water. The effects of the constraint types of clamp and the pressure holding time and temperature on pressure variation were also investigated. The results indicated that there was no effect from the constraint types on the pressure, but the increase of pressure holding time could effectively suppress the pressure decrease and decline rate. As a result, the test stability was improved. The pressure in the hydrostatic test of PE-HD was reduced with an increase in temperature, and there was a linear relationship observed between the temperature and the decline rate of pressure.