Disclosed is a method for preparing a three-dimensional porous nano composite cooling film in large scale. The cooling film is prepared from 0.1-0.5 parts of cellulose acetate, 1-5 parts of water, 20-100 parts of acetone, an additive, and 10-20 parts of nano microspheres through a cooperative formulation of cellulose acetate, nano microsphere materials and the additive. The composite cooling film is obtained by self-deposition of cellulose acetate and nano microspheres, and liquid volatilization during film forming process leads to formation of the three-dimensional porous structure. The film has an effect of enhancing radiation of infrared heat into space, which could significantly reduce a temperature of a substrate surface and achieve rapid and strong cooling. The film could achieve effective cooling without external power and other active cooling equipment, with or without sunlight.