Abstract: The mechanism of hydrostatic pressure affecting the irradiation swelling of UO₂ high burnup structure was analyzed. Three basic assumptions used to develop the constitutive equation of irradiation swelling were made accordingly. It is concluded that hydrostatic pressure imposes an important impact on irradiation swelling mainly through compressing the UO₂ high burnup structure pores. Based on the already developed correlation of the irradiation swelling of UO₂ high burnup structure, pore shrinkage due to the application of hydrostatic pressure and thus the reduction of irradiation swelling of UO₂ high burnup structure were determined quantitatively, and the constitutive equation of irradiation swelling of UO₂ high burnup structure considering the hydrostatic pressure was constructed successfully. The constitutive equation is validated using available irradiation swelling data of UO₂ high burnup structure, which demonstrates its reasonability.