The effects of exhaust pipe length between the diesel engine turbocharger outlet and SCR device on exhaust temperature affected the efficiency of SCR. Effects of exhaust pipe length on exhaust temperature were conducted on the engine test bench. Based on the emission cycles of ETC and WHTC, the influences of exhaust pipe length and wrapped heat preservation material on exhaust temperature were analyzed. The test results showed that a long length of exhaust pipe would lead to a great drop of exhaust temperature and hence had a great influence on NO_xemission. The maximum drop of exhaust temperature was as high as 19% and the NO_xemission was also 59% higher than the set limit. The heat preservation material could prohibit the heat release. The maximum temperature drop was 8% and the NO_x emission increase by 23% after the insulation work.