Abstract: By adding a gas injection system to the intake manifold, a marine engine is converted into a dual-fuel diesel/CNG engine. Bench tests are performed in fuel mode, fuel mode with SCR and dual-fuel mode to explore its emissions and economy. The results show that the NO_x emission of the engine in dual-fuel mode can reach the emission level of the fuel mode with SCR, which meets the Tier Ⅲ of MARPOL Annex Ⅵ for NO_x emission. However, the manifold intake results in high HC and CO emission, which makes it difficult to meet the emission standard for marine engines in China（GB 15097-2016）. MARPOL Annex Ⅵ does not impose any restrictions on dual-fuel diesel/CNG engines with high HC and CO emissions, an omission that needs to be reviewed and improved. Both the engine fuel mode with SCR and the dual fuel mode meet Tier Ⅲ standards. However, the dual-fuel mode is more cost effective than the engine-fuelled mode when the ship on which the engine is installed navigates in the MARPOL Annex Ⅵ emission control area. The results of the study are expected to serve as a reference for ship operation management and the revision of MARPOL Annex Ⅵ.