In order to avoid the urea crystallization on urea nozzle or mixer wall of conventional SCR catalyst under low load conditions with low exhaust gas temperature, the structural optimization of SCR catalyst was conducted with CFD method and a newtyped SCR mixer was designed. The corresponding distribution pattern of urea nozzle was also proposed. The simulation results show that the optimized SCR converter can promote the mixing between urea solution and exhaust gas. The thermolysis rate and concentration distribution uniformity index of NH₃ for fore end face increases by 9.76% and 6.06% respectively. A series of verifications such as steadystate test, ETC test and 30hour urea crystallization test are conducted on the test bench. The test results show that the new structure can increase NO_x conversion rate by 0.89%, 1.76% and 0.61% respectively when the ratio of ammonia and nitrogen is 0.7, 0.9 and 1.2. Compared with the original structure, the new structure can significantly reduce NO_x emission and avoid urea crystallization on the wall of new mixer.