The invention relates to an energy-saving cast-in-place hollow floor, which comprises thin-wall box bodies, a rib beam between the box bodies and an upper flange plate, wherein the upper flange plate contains an electric heating tube, and an electric heater and heat retention liquid are arranged in the electric heating tube; the top surface of the thin-wall box body is provided with at least one groove, the lateral surface of the thin-wall box body is provided with a closed hole, the closed holes of the adjacent box bodies are connected through a junction pipe, the lower end of the hole wall is connected with the bottom surface and closed, and the upper end of the hole wall is communicated with the top surface to form a hole; and the inclined plane and the bottom surface of the box bodies form an included angle of 100 to 165 degrees, and the side length of the inclined plane is more than 20 millimeters. Sunken closed holes are reserved on the lateral surfaces of the box bodies, the closed holes are opened before casting and the junction pipe can pass through the open closed holes to form an interconnected through heat energy blowing passage, and grooves for laying heating pipes are reserved on the top surfaces of the thin-wall box bodies to form a floor energy supply system. The floor adopting the thin-wall box bodies has the advantages of light weight, high hardness, reasonable structure, energy conservation, environment friendliness, cracking resistance and seismic resistance, and also can solve the heating problem by using the box bodies to transfer the heat energy.