The invention discloses a preparing method of an exterior wall external heat insulation material of a geopolymer-based polyphenyl particle building and relates to the field of building exterior wall external heat insulation materials. The exterior wall external heat insulation material is characterized by containing 30-50% of geopolymer cementing materials, 39-60% of modified polyphenyl particles, 1-3% of redispersible latex powder and 0-1% of methyl celluloses according to weight percentage. In the preparing process, polyphenyl particle surfaces are modified, modified polyphenyl particles of a core-shell structure are prepared, compatibility of the modified polyphenyl particles and other geopolymer cementing materials is effectively improved, and accordingly thermal insulation mortar comprehensive performance is improved. The geopolymer cementing materials replace the traditional cement so that the exterior wall external heat insulation material of the geopolymer-based polyphenyl particle building has the advantages of being environment-friendly and energy-saving in production process.