Disclosed are a system and a method for promoting the formation of the emulsified fuel of a diesel engine. The system includes a viscous oil-water tank; an emulsified fuel mixing device for mixing viscous oil-water with diesel fuel to obtain emulsified fuel; a dual-fuel return fuel emulsification-promoting device connecting to a diesel tank, the emulsified fuel mixing device, and the return fuel pipe and the main fuel pipe of the engine respectively; a heat-exchanging device for heating the emulsified fuel from the dual-fuel return fuel emulsification-promoting device by the cooling water of the engine; a first purifying pipe and a second purifying pipe for purifying the main fuel pipe; and a dual-fuel management device for switching between the diesel fuel supply and the emulsified fuel supply according to the temperature of the cooling water, and after the ignition switch of the engine is closed, for controlling the diesel fuel to clean up the emulsified fuel in the engine, in the main fuel pipe and in the devices. By using the system, the water content of the emulsified fuel can be up to 33% and the diesel engine can keep essentially unchanged power, start easily and run steadily.