In order to study the main influencing factors of direct sampling results for engine dynamic test, a diesel engine was used to compare direct and fullflow constant volume sampling results by changing different test parameters based on the theoretical analysis. The results showed that the fullflow constant volume sampling test results were less affected by the changes of test parameters. Whether the realtime concentration of direct sampling exhaust aligned with the exhaust mass flow or not according to conversion time was the key factor for the accuracy of emission test results. The change of exhaust pipe length would lead to different conversion time, which could be corrected by testing CO2 delay. Compared with the fullflow constant volume sampling, the dispersion of direct sampling test results were large at the low direct sampling frequency. The change of pipe length after air flow meter had minor influence on the direct sampling test results.