In order to study the influence of arc board structure on overall performance of cooler of air coolermotor，according to the theory of Computational Fluid Dynamic( CFD) and Numerical Heat Transfer( NHT) ，a YX-KK710 － 4，4 000 k W large efficient asynchronous motor was taken as an example． The physical and mathematicalmodel of cooler were created，and basic assumptions and boundary conditions were determined． The coupled fieldof cooler was calculated and analyzed． Based on the length of the arc board region as a constant，the flow shows that the vortex become less obvious than the original cooler’s，and the cooling performance was enhanced． A new cooler was manufactured according to the numerical result，the rated volume flow of original cooler and new coolerwere measured，then the flow distribution of all cooling pipes was measured and analyzed carefully． The tempera-ture of every outlet of cooler was measured and compared with the result of numerical calculation． The experimentsshows that the new cooler has a better overall performance，and it can provide theoretical basis and new method foroptimization design of efficient motor．