The invention discloses a novel external wall heat insulating foaming putty, which is mainly formed by the following materials with percentage by weight, 15-25% H2O, 0.05-0.15 preservative agent, 0.8-1.5% acrylate, 0.2-0.5% dispersing agent, 1-1.5% anti-freeze agent, 0.5-2% NH4 HCO3, 1-2% 12 carbon betulin, 3-5% wood fiber or polypropylene fiber, 0.4-0.6% water-loss reducer, 10-20% propyl benzene emulsion, 20-30% CaCO3 powder, 30-40% talcum powder, 3-5% polyphenylene particle and 0.5-1.5% wax emulsion. The external wall heat insulating foaming putty adopts the self-foaming principle, provides heat insulating effect through adding polyphenylene particle in heat-insulating mortar, has excellent heat insulating effect, and can be extensively applied on the external walls of buildings.