Abstract: Based on CFD technology, the flow field characteristics of a general purpose engine are analyzed by fluid dynamics simulation software STAR-CCM+ 11.06, and the effects of the opening of the fan cover, the opening of the bottom of the box, the inner baffle and position of the fan cover on the flow field characteristics and air distribution are studied. The results of six different structural improvement programs show that the flow field distribution characteristics of the general purpose engine under different structures are basically the same as that of the original structure, and the average cell relative velocity of the engine surface is almost the same. The improved structure with the hole area at the bottom of the engine crankcase increased from 240 mm² to 320 mm² and the baffle in the right part of the fan hood moved up 5 mm has better flow field distribution characteristics. The air volume distribution of the improvement design at the bottom of the engine crankcase, the middle of the cylinder head and the cooling ducts near the exhaust duct is slightly better, and the total air volume at the fan inlet of the design five is significantly increased, which is 6% higher than that of the original structure, which is conducive to the improvement of the cooling performance of general purpose engine. The flow field analysis and the flow field characteristic evaluation of the structural scheme design provide the theoretical support for the early design and development of the follow-up products and the system matching work procedures.