Abstract: Abstract: The opening of a variable nozzle turbocharger (VNT) has a significant impact on the boost pressure, torque response, and emissions of a diesel engine under transient conditions. In order to achieve better diesel engine transient performance, realize the coordinate control of the diesel engine and the VNT nozzle opening and to improve the transient response of the diesel engine while reducing the emissions. In this study, the VNT opening was calibrated on the bench of a high-pressure common-rail diesel engine for better performance. A bench test was conducted to investigate the effect of VNT openings on the transient performance and emissions of diesel engine at the rotation rate of 1 600, 2 000, and 2 600 r/min, when the response load was transient from 0 to 50%, 75%, and 100% at 1, 3, and 5 s of an accelerator pedal. The results show that there was a much more obvious effect of VNT opening on the transient condition of load response time 1, 3, and 5 s from 0 to 50%, 75%, and 100% of the load at 2 000 r/min than that of 3 and 5 s. The response time of boost pressure gradually decreased, while the response time of torque decreased and then increased, as the VNT opening decreased. The NOx volume fraction first increased, then decreased, and finally increased to a stable value, when the response time of accelerator pedal at 1s from 0% to 50%, 75% and 100% at 1 600, 2 000 and 2 600 r/min. The particle number (PN) emission increased gradually with the decrease of VNT opening. A World Harmonized Transient Cycle (WHTC) test was carried out after the optimum VNT opening was obtained at different speeds. The simulation data showed that the VNT opening reduced the intercept and standard deviation for the WHTC cycle torque regression line. The intercept decreased by 0.03 and 0.37 in cold and hot states, while the standard deviation decreased by 0.11 and 1.07, respectively. The NOx brake specific emissions decreased by 7.59% and 2.21% under the WHTC cold and hot cycles, while the particulate matter (PM) brake specific emissions reduced by 8.64% and 25.28%, and the PN brake specific emissions dropped by 6.74% and 12.4%, compared with the original engine. After optimizing the VNT opening, the standard deviation of the actual torque value from the reference value is reduced, and the main pollutants are reduced in different proportions, which shows the feasibility of the coordinated control of the VNT opening to optimize the transient performance and emissions of the diesel engine. The finding can provide a sound reference for the coordinated control of VNT opening under transient operating conditions in a diesel engine.