Abstract: Creep tests for domestic RPV steel, A508-Ⅲ, have been performed under three types of load, 17.5MPa, 20 MPa and 27 MPa at 800℃. Furthermore, the staged creep tests of A508-Ⅲ steel under 20 MPa and 27 MPa are carried out. The study on the microstructure and creep curve shows that the volume fractions of cavities and second phase particles increase linearly in the process of creep, so the nucleation and growth of cavities as well as second phase particles coarsening have become the main reason of creep damage. Based on mesomechanics ideas and analysis on the microscopic creep damage mechanism of A508-Ⅲ steel at 800℃, an K-R creep damage constitutive equation involved microscopic damage and its evolution is established through defining a representative volume element, which is a three-phase composite volume consists of no damage phase, cavities phase and second phase particles. By the normalization process, the creep constitutive equation and damage evolution equation reflected the evolution of cavities and the second phase particles are obtained, which set up the inner relation between micro structure damage and macroscopic constitutive equation.