By using a portable emission measurement system(PEMS), the road-based emission tests were conducted on three heavy-duty diesel vehicles. The engine load rate and CO2 emission rate were used as the reference for dividing the engine into low load or high load conditions. Combined with specific emissions, transient emissions and etc, the emission characteristics of CO, CO2, NOx, PN23 under low load conditions and during cold start process were analyzed, and the variation law of PN10 specific emissions under low load conditions was explored. The research results indicate that the low load condition based on CO2 emission rate is more representative. Under low load conditions, the specific emissions of CO, CO2, NOx, and PN23 are higher, and CO, NOx, and PN23 are prone to exceed the standard limits. There are a large number of low load working points with high CO and NOx specific emissions during the cold start process. Furthermore, reducing the lower limit of PN counting particle size from 23 nm to 10 nm will result in a doubling of PN10 specific emissions under low load conditions.