Abstract: In accordance with the MARPOL Annex Ⅵ Nitrogen Oxide（NO_x） Technical Code issued by the International Maritime Organization（IMO）, this paper proposes a simplified on-board test scheme for measuring NO_xemissions from marine diesel engines. The scheme employs the carbon balance method to calculate exhaust flow, requiring only the measurement of NO_xand CO₂concentrations in diesel exhaust at selected load points, and adopts the default fuel element content values provided in the IMO Technical Code. The results show that the weighted average NO_xdeviation of the simplified method is approximately 2% compared with the standard cycle, while also reducing the complexity of emission testing equipment and shortening the testing process. The findings provide a theoretical basis for streamlining NO_xemission on-board tests.