The invention provides a polyurethane and foamed cement microscopically-composite heat insulating material and a preparation method therefor and belongs to the field of organic / inorganic composite type building materials. The preparation method is characterized by comprising the steps: adding ingredients such as cement and a foam stabilizer according to the conventional method, preparing foamed cement through carrying out foaming with hydrogen peroxide, and then, carrying out room-temperature curing until the foamed cement reaches an age; and then, mixing isocyanate, polyether polyol, polysiloxane polyether and a catalyst so as to obtain uniform slurry, then, submerging the dried foamed cement in the slurry, taking out the foamed cement after the foamed cement is completely impregnated with the slurry, carrying out standing for 2 to 4 hours so as to cure the foamed cement, then, submerging the foamed cement in water again, and then, carrying out foaming in a manner that water molecules are in contact with incompletely-reacted isocyanate through diffusion and infiltration, thereby obtaining the microscopically-composite heat insulating material. According to the material, organic polyurethane foam and inorganic cement foam are mutually independent and coexist in an interpenetrating manner, and microscopic compounding among interfaces is achieved, so that the foamed cement has relatively high compressive strength and relatively low coefficient of thermal conductivity.