Abstract: A Cummins 6C diesel generator unit (S688CCS) is taken as the research object in this paper and the fluid dynamics analysis software STAR-CCM+ is applied for conducting the numerical simulation study on flow field distributions of its conventional type and silent type. The difference of fluid distribution characteristics between conventional type generator unit and silent type generator unit is compared. Simulation results indicate the total cooling air of the silent generator is lower than that of the conventional generator, and the cooling air at the radiator is also lower than that of the conventional generator, which is not good for generator cooling. Based on the results of flow field comparison analysis, single variable control method was adopted to improve the structure of fan cover, silent components for silence, adjacent structure and air inlet grille. The flow field distribution characteristics were optimized, and the corresponding structure optimization scheme is put forward. After optimization, the cooing air amount flow through radiator of the silent generator unit is 44.33% higher than that of the radiator of its original structure, and the total amount of cooling air is higher than that of the conventional unit. The research results in this paper can provide a theoretical basis for the design of cooling air flow-path of generator units.