The invention provides a method for treating cotton field straw and residual biodegradable mulch films.The method comprises the following steps that before autumn ploughing of a cotton field, the straw and the surface residual biodegradable mulch films are collected, and the smashed straw and the residual biodegradable mulch films are loosely stacked together and are not treated any more in the current year; composting treatment is carried out and completed from 25th May to 10th September of the next year, the cotton field straw and residual biodegradable mulch films stacked in the last year are evenly mixed with waste fungus bag residues or saw dust and animal manure in proportion, and a proper number of biological bacteria are mixed; stack turning and humidification are carried out, and fermentation is started when water content in the materials reaches the level that water stains are left on the hand and no water drop is pinched out when the materials are pinched by hand; the water content is controlled, stack turning is carried out in time, and the humidity of the material stack is controlled; the materials are covered when it rains in the composting process, and composting is completed in 40-60 days.According to the method, bacteria and pests of the straw can be killed, and increase of the cardinal number of cotton field bacterium sources and the population cardinal number of pests is effectively restrained, so that the straw is returned to the field safely and effectively, and meanwhile all the residual biodegradable mulch films are degraded.