The emission characteristics of a heavy-duty diesel hybrid vehicle were researched on the heavy-duty chassis dyno in the lab by using CHTC-D test cycle based on hybrid mode and pure engine mode respectively. The results show that the exhaust temperature of hybrid mode fluctuates when the engine starts and stops frequently and has a step-type sudden change especially in low-speed section. Compared with the pure engine mode, CO emission of hybrid mode is lower，the comprehensive specific emission reduces by about 50.9% and reaches the reduction of 78% especially in the low-speed section. The NO_x and PN emissions both increase significantly, NO_x comprehensive specific emission increases by about 2.0 times and increases significantly in high-speed section, and PN comprehensive specific emission increases by about 4.4 times and increases significantly about 12.4 times in the low-speed section. Accordingly, the hybrid mode has the obvious advantages in carbon emission reduction with the total reduction of  about 70.1%.