Abstract: In order to evaluate the flow field characteristics of a generator unit, based on the CFD method, the simulation model of the external flow field of the generator unit is established by fluid analysis software STAR-CCM +, and the air volume distribution, component velocity field and section velocity field are comprehensively analyzed. The results show that the fan outlet has a large air velocity, which is guided to the cylinder head and crankcase air duct through the air guiding effect of the fan cover to cool the engine. The maximum air velocity at the engine heat sink can reach 10 m/s, and the air velocity at the bottom of the engine crankcase is evenly distributed, with the air velocity of 6 ~ 8 m/s. The overall flow field velocity distribution of the unit is uniform, and there is no obvious flow dead zone. The air velocity on the surface of the engine (cylinder head, crankcase bottom and crankcase body) is evenly distributed without hot air reflux. The air velocity distribution on the surface of stator and muffler is good, which is conducive to the cooling of high-temperature parts. The air inlet layout of generator unit is discussed. After changing 5 air inlets to 2 air inlets, the total air inlet volume of the unit decreases from 75.07 g/s to 62.12 g/s, 12.95 g/s decreased and the decrease ratio is 17.3%. The air velocity on the surface of frequency converter and motor stator increases significantly, the air velocity on the surface of engine and muffler decreases, and the air outlet layout has a great impact on the flow field of the generator unit. The position of air inlet and outlet shall be reasonably selected according to the design requirements of generator unit cooling system to ensure the cooling of generator unit. The research results can provide simulation data support and theoretical reference for the design and structural improvement of generator cooling system.