The invention relates to a phase-transition composite material which comprises: A) 30-65% of phase-transition materials which are low melting point paraffin with a melting point of 25-45 DEG C and/or dodecanol; B) 25-45% of carrier materials which are high density polyethylane and/or ethane-vinylacetate copolymer; C) 5-15% of inorganic fillers which are porous substances and are selected from one or two of expanded perlite and expandable graphite; D) 1-10% of heat-conduction reinforcing agents; and E) 1-10% of fire retardants. The composite material has an appropriate phase transition temperature which matches the building ambient temperature, has great phase transition latent heat, excellent heat-preservation and heat-insulation performance, is fireproof and flame-retardant, is easy to process, and has greatly reduced phase-transition material leakage. The invention also relates to a method for preparing the phase-transition composite material, which comprises the thermal insulation mortar of the phase-transition composite material and a method of heat preservation of a wall by using the mortar.