Ensuring lower emissions and better economy (fuel economy and after-treatment economy) simultaneously is the pursuit of future engines. An EGR-LNT synergetic control system was applied to a modified lean-burn CA3GA2 gasoline engine. Results showed that the synergetic control system can achieve a better NOx reduction than sole EGR and sole LNT within a proper range of upstream EGR rate and without the penalty in fuel consumption. It also has the potential to save costly noble metals in LNT, but excessive or deficient upstream EGR would make the synergetic control system inefficiency.In order to guarantee the objectivity of the effect of EGR-LNT synergetic control system on NOx reduction, another modified lean-burn CA4GA5 gasoline engine was additionally tested. Results verified that proper range of upstream EGR rate in EGR-LNT synergetic control system and showed that, less upstream EGR could not lower the NOx generated from in-cylinder effectively, while more upstream EGR would deteriorate NOx conversion efficiency of downstream LNT, either of these two cases will affect the final application effect of EGR-LNT synergetic control system.The emissions of those modified lean-burn gasoline engines were specifically analyzed. CO2, H2O(g) and O2 were proved to be the major gases that vary with upstream EGR at the inlet of downstream LNT. A following experimental study of the effect of CO2 and H2O(g) on downstream LNT was further conducted on the modified lean-burn CA4GA5 gasoline engine. Results showed that, compared with H2O(g), CO2 is the major factor in affecting NOx conversion efficiency of LNT. H2O(g) together with CO2 can strengthen the deteriorative effect at temperature over 300 °C and it only can affect NOx conversion efficiency of LNT independently at temperature lower than 300 °C.