The reaction path of urea decomposition was analyzed from the perspective of molecular structure chemistry. The detailed chemical reaction model of SCR urea injection system including 14 steps and 11 components was constructed based on the path analysis and previous research experience, which considered both the formation of ammonia and isocyanic acid and the formation of important solid by-products such as biuret, cyanuric acid and cyanuric acid monoamide. The problem that the kinetic parameters of activation energy and pre-exponential factor in the two reactions of model that cannot be obtained by literature was solved according to the quantum chemical calculation method. A three-dimensional model was built for the decomposition process of urea in the exhaust pipe from the injection point to the front end of catalyst inlet. The detailed mechanism model and the total package reaction mechanism model were introduced respectively. The calculation results showed that the trend of NH3 concentration generation predicted by the two mechanism models was consistent. The formation of biuret and cyanuric acid monoamide was the main reason for the prediction error of NH3 concentration at about 200 ℃.