In order to perfect the work efficiency evaluation system of diesel SCR system and improve the work efficiency of SCR system, the theoretical urea decomposition efficiency calculation method of diesel engine SCR system was proposed. The urea decomposition efficiency, amount of leakage of NH3 and NOx conversion efficiency of SCR system for diesel engine were investigated on an engine test bench, the influence of space velocity, NH3/NO ratio and SCR temperatures were analyzed. Experiment results showed that the changing trend of NOx conversion efficiency and urea decomposition efficiency was basically the same. In the catalyst activation temperature range, the urea decomposition efficiency was above 90%; with the increase of space velocity, the urea decomposition efficiency was increased first and then decreased. NH3/NO ratio was 1 with high NOx conversion efficiency, but the amount of leakage of NH3 was exceeded the limit value. NH3/NO ratio had little effect on the urea decomposition efficiency, and it was increased with the amount of leakage of NH3 increasing. When the exhaust gas temperature was raised, the urea decomposition efficiency increased. The space velocity and exhaust gas temperature were the most important factors influencing the urea decomposition efficiency, the level of urea decomposition efficiency was the comprehensive effect of both. Through the study on the factors affecting the decomposition efficiency of urea, to reduce the deposition of urea in the exhaust pipe and improve the removal efficiency of NOx. The proposed calculation method can exhibit a trend of urea decomposition efficiency and provide a basis for improving the work efficiency of SCR system. 