Abstract: Taking a model of off-road engine as the test engine, and the reaction section was a SCR catalyst with two sections of vanadium-based extruded catalysts of 7″ diameter, 200 mesh, 25 mm and 50 mm in length. The ammonia generation characteristics inside the catalyst were investigated, and explored the effects of structural parameters and operating conditions on the urea decomposition rate and ammonia distribution inside the catalyst. The results showed that with the increase of exhaust gas temperature and the decrease of flow rate, the uniformity of ammonia distribution in the radial measurement cross-section of the catalyst and the efficiency of SCR reaction was increased.Under each test condition and structural condition, urea decomposition mainly occurred in the first 100 mm of the catalyst, and the decomposition rate was faster in this length and gradually slowed down. At 150 mm catalyst length, the average decomposition rate of urea at the catalyst outlet was more than 80% under all conditions, which could meet the requirements of standard SCR reaction. In this paper, the study of urea decomposition characteristics and ammonia distribution through bench test can provide some reference for improving the efficiency of urea utilization and optimizing the catalyst design of after-treatment system.