The invention relates to a reusable concrete floor steel bar pre-supporting device, which comprises four upright posts, a square positioning steel plate, hanging arm supporting arms, steel bar hanging arms and pulling arms. Due to the structure, the device is processed in a factory to ensure processing accuracy, and is suitable to be used at the cross junction of steel bars, and a floor steel bar layer is accurately positioned; L-shaped right-angle steel bar hooks are located at opposite positions by pulling the pulling arms, so that the lifted steel bars are oppositely fixed and cannot slide; after concrete is poured, the L-shaped right-angle steel bar hooks are rotated to be located at withdrawable positions by pulling the pulling arms during surface finishing, and the steel bar pre-supporting device is taken out, cleaned and reused; and meanwhile, the extension length of the steel bar hanging arms can be adjusted according to the thickness of a floor, the four upright posts with appropriate length can be replaced when the adjusted steel bar hanging arms cannot meet the actual requirement. The device has a simple structure, is convenient to assemble and disassemble and can be rented and turned over, construction cost is reduced, and the input of construction personnel is reduced.