The kinetics of U(IV) produced by hydrazine reducting U(VI) with platinum as catalyst in nitric acid media was studied for revealing the reaction mechanism and optimizing the reaction process. Electron spin resonance (ESR) was used to determine the influence of nitric acid oxidation. The influence of nitric acid, hydrazine, U(VI) concentration, catalyst dosage and temperature on the reaction were studied. Concurrently, the simulation of the reaction process was performed using density functional theory (DFT). The results show that the influence of oxidation on the main reaction is limited when the concentration of nitric acid was below 0.5 mol/L. The reaction kinetics equation below the concentration of 0.5 mol/L is found as follows: –dc(UO22+)/dt) = kc0.5323(UO22+)c0.2074(N2H5+)c−0.2009(H+). When the temperature is 50°C, and the solid/liquid ratio r is 0.0667g/mL, the reaction kinetics constant is k = 0.00199 (mol/L) 0.4612/min. Between 20℃ and 80℃, with the increase of temperature, the reaction rate is gradually accelerated, and the reaction changes from chemically controlled to diffusion-controlled. The reaction process is simulated by DFT, and the influence of various factors on the reaction process is deduced. The reaction process and mechanism are determined according to the reaction kinetics and simulation results finally.