The invention discloses a stock garbage screening and reduction treatment system and method. The stock garbage screening and reduction treatment system comprises a gas injection device, a gas extraction device, a methane concentration detection device, an airing system, a crushing device, a screening device, a sorting device, a winnowing device, a magnetic separation device and a leachate collecting device. The gas injection device injects inert gas into a stock garbage heap, meanwhile, the gas extraction device extracts landfill gas in the stock garbage heap, the concentration of methane in the stock garbage heap is reduced, and the excavation safety is improved. The airing system can reduce the water content of stock garbage, and the follow-up screening effect is improved. The treatment method comprises the steps that the stock garbage is aired before being screened, the water content of fed materials is controlled to be 30% or below, and the screening effect is remarkable; and the stock garbage is sequentially subjected to airing, crushing, screening, sorting, winnowing and magnetic separation, the treatment direction of screened products is reasonable, the project production halt risk caused by unsmooth discharging of the screened products is avoided, and meanwhile the problem of secondary pollution in the outward transportation process of the screened products can be avoided.