Abstract: Heavy-duty diesel engine emission regulations continue to reduce nitrogen oxide（NO_x） limits,and using close-couple selective catalytic reduction（SCR） is one of the expected technical routes. However,whether it is necessary to use a close-couple diesel oxidation catalytic（DOC） to improve the NO₂ ratio at the inlet of the closecouple SCR still needs further investigation. The DOC and SCR small sample models were respectively developed in this study based on GT-Power platform for a commercial diesel engine. Initial calibration of reaction kinetic mechanism parameters based on small sample test data and genetic algorithms,a DOC-DPF-SCR-ASC combined system model was built by preliminary calibration,and then using the test cycle data to correct the mechanism parameters. Based on the system model,the necessity of using close-couple DOC was discussed,and the results show that close-couple DOC will increase the thermal inertia before the close-couple SCR and reduce the cold start performance of the close-couple SCR. A volume matching study between the two-stage SCRs was also carried out.When the main SCR volume is 9.94 L,matching the close-couple SCR with a volume of 2.13 L can make the SCR conversion efficiency under cold start to increase from 63.0% to 77.8% and NO_x is decreased by 42.4%. Weighted NO_x specific emissions obtained on hot and cold world harmonized transient cycle（WHTC） operating cycles are decreased from 0.38 g/（k W·h） to 0.21 g/（kW·h）,a decrease of 44.7%.