The invention relates to a hollow solar heat-collecting flat box. On the premise that a heat storage water tank and a lighting heat-insulating box body fixedly connected with the heat storage water tank are inclined a certain angle backward and inclined a certain angle sideward simultaneously, a heat collector is made into a large hollow flat box by a thin metal plate. The flat box consists of an upper box cover and a lower bottom plate which are combined, wherein a plurality of ribs are arranged on an upper panel and a lower panel of the flat box by pressing; bolts and nuts matched with the bolts are arranged at the central positions of the upper panel and the lower panel of the flat box to connect the upper panel with the lower panel toward the inside of the box in a pulling and pressing mode, so that the upper panel and the lower panel are sunk toward the inside of the box. Under the combined action of the pressed ribs and the bolts of pulling and pressing the upper panel and the lower panel, the heat-collecting flat box cannot be expanded outward and deformed a lot when filled with water; meanwhile, when the water in the heat-collecting flat box is frozen into ice to cause volume expansion, the flat box can be subjected to secondary expansion deformation to ensure that the box body cannot be broken through expansion.