This paper adopts harmonic reaction method to calculate the cooling load of the air conditioning system of the NO.1011A loaded miniature cars in Chongqing Chang'an Automobile Company .The study indicates that harmonic reaction method is more exact than traditional steady method to show the dynamic characteristic of cooling load. The maximum cooling load appears on 2:00 pm, it's 3 098 W. Among the whole cooling load,heat load through windows and heat load through components are the biggest,they occupy 50% of the whole amount. Heat load of air leaks occupies 27.3%,which shows that it is necessary to improve the seal of cars. Chongqing Chang'an Automobile Company develops optimize design for the air condition system with this result, which reaches a perfect climate effect.