Based on the PEMS, three road tracking emission tests in different vehicle mileage were conducted on a heavy-duty diesel truck with DOC and CDPF. Test results showed that CO, THC and PM emission factors of original vehicle showed a downward trend, while NO_x and PN showed an upward trend from low speed to high speed condition. After installing DOC and CDPF devices, the emission reduction rates of CO, THC, PM and PN all increased gradually with the increase of vehicle speed, but the NO_x emission almost kept the same. With the increase of vehicle mileage, the DOC and CDPF device's performances of reducing CO and NO_x emissions became worse. The maximum reduction rates of CO and NO_x appeared in the first test, which was 59.8% and 9.7% respectively. The device's performances of reducing THC, PN and PM emissions were first good and then became worse. The maximum reduction rates for three pollutants appeared in the second test, which was 53.5%, 99% and 87.3% respectively.