The invention discloses a thermal-insulation paint for building external walls, which is prepared from the following raw materials in parts by weight: an epoxy resin, bismaleimide, di-tert alcohol propenyl borate, foam glass, an ethylene-vinyl acetate copolymer, 3,3'-(p-phenyleneoxy)bis(2,4,5-triphenyl)cyclopentadiene ketone, nano alumina-silica fiber, a dispersing agent, a thickener, a defoaming agent and a wetting agent. The paint has the advantages of low raw material loss, simple technique, hard paint film, favorable glossiness, high impact resistance, low cracking tendency, soft tactile sensation, high decorating property, high weather resistance, high scrub resistance, high adhesive force, long service life, no special smell, no heavy metals (such as mercury, lead and the like), no volatile formaldehyde, synergistic actions among substance molecules, very high stability, high adhesiveness and high film formation efficiency, enhances the density of the coating film, and improves the film formation quality. Thus, the film has the advantages of high adhesive force, no bubble or crack, high compactness, high uniformity and the like.