Abstract: The flow field analysis and cooling performance evaluation of the cooling system of the generator unit are helpful to confirm the total flowrate of cooling system, the distribution proportion of cooling air volume and the velocity distribution characteristics of cooling air, so as to ensure uniform flow distribution in the air duct and good cooling in the high-temperature area. Taking an open frame generator unit as the research object, the initial layout and structure design of the cooling system are designed according to the key design points of cooling system, and the flow field analysis model of the cooling system of the open frame generator unit is built by fluid flow analysis software STAR-CCM+. The results show that the air volume of cylinder head air duct accounts for 48.77% of the total air volume of engine air duct, and the air volume at the bottom of engine crankcase accounts for 6.7%, which is not conducive to the cooling of engine cylinder head and engine oil. The structural improvement design of the cooling system of the open frame generator unit is formed by adjusting the spacing between the deflector and the heat sink, changing the fan cover to the form of gradually expanding line, changing the matching between the hole opening and the shape of the heat sink and the diversion, and increasing the air inlet area through adding more openings on pull plate. After structure improvement, the air volume of the cylinder head air duct of the unit accounts for 61.83% of the total air volume of the engine air duct, and the air volume at the bottom of engine crankcase accounts for 13.04%. The air volume of cylinder head air duct and the air volume at the bottom of engine crankcase basically meet the cooling requirements of engine cylinder temperature and oil temperature. The cooling air velocity of each air duct of the engine is evenly distributed, the motor is well cooled, and the air inlet of the air filter comes from the outside fresh cooling air. The air velocity on the muffler surface is evenly distributed, there is no flow dead zone, and the air velocity near the exhaust pipe area is large. The cooling system of the generator unit can realize better cooling of the unit and ensure the normal and long-term operation of generator unit. The research results can provide simulation data support and theoretical reference for the layout design and structure improvement of generator unit cooling system.