Abstract: In this paper, for the goal of sintering the uranium dioxide pellet in low temperature, UO₂ powder was surface pre-oxidation treated by thermal analyzer, then the transformation point, specific area, phase component and microstructure of powder were studied by specialized instruments. The results show that there is a little of U₃O₇ in uranium dioxide powders by pre-oxidation treatment at 240℃ for 8h in air, and UO₂ powders will transform into U₃O₈ at 382℃for 8h in air. The simulation sintering of UO_2+x pellet was launched by the thermal dilatometer. Shrinkage temperature of the uranium dioxide pellets after pre-oxidation treatment decreased from 1200 ℃ to 580 ℃, and the densification rate（ΔL/L） increased also from 1.52×10^-4/K to 3.08×10^-4/K. The mechanism of low temperature sintering to pre-oxidation UO_2+x pellets was explained by the simplified point defect model and densification equation. The UO_2+x diffusion coefficient is much higher than that of UO₂. The densification factor A is positive correlation to UO_2+x diffusion coefficient,and it could be expressed by aquadratic polynomial according to temperature T, which factor a=16.23658,b=0.04247,c=-2.18802×10^-5.