The invention discloses an assembled integral building and a construction method thereof. The assembled integral building comprises a reinforced concrete wall and floor slabs, wherein the reinforced concrete wall is formed by hoisting and assembling prefabricated composite insulating wallboards integrally at a construction site; a board body of each prefabricated composite insulating wallboard comprises a structure layer, an insulating layer and a protective layer from the inside to the outside in turn; a reinforcement cage is arranged in the structural layer; steel meshes are arranged in the concrete of the protective layer; the structural layer, the insulating layer and the protective layer are anchored together by inserting connecting members in a dotted distribution; the left and right edges of the protective layer and the insulating layer protrude out of the structural layer; a lower edge of the protective layer extends downward to form a bulged opening; the top parts of the protective layer and the insulating layer upward protrude out of the structural layer; and an upper edge of the protective layer is provided with a notch which corresponds to the bulged opening of the prefabricated composite insulating wallboard adjacent to an upper layer. The building overcomes the defects of erecting shuttering on site, binding steel bars and concreting in a traditional construction method, speeds up the construction, and lowers the construction cost; and the construction quality of the building structure can be sufficiently guaranteed.