The invention discloses a device and a method for launching after flat ground construction of a self-elevating platform. The device comprises a concrete box body foundation 1, closed steel box bodies 2, hollowed-out steel structure supporting units 3 and steel structure construction platforms 4. All components of the device disclosed by the invention are hermetically and fixedly connected, wherein the closed steel box bodies 2 have a ballast adjusting function, water can be filled into a buoyancy box during use to enable the buoyancy box to sink down to a seabed base and the drainage of water in the buoyancy box is finished during removal to enable the buoyancy box to float and move. After the platform is constructed on the land of a wharf, the device is divided into a plurality of different units according to the features of the platform, the different units are configured on the seabed of the wharf in a manner of filling ballast water, fixed sliding rails are laid on the wharf, the platform is moved to sliding rails on the sea, then the entire platform is elevated, the sliding rails are removed, finally the ballast water in the buoyancy box is drained and the buoyancy box floats and moves away. The device can replace the expensive semi-submersible barge or floating dock, has the features of high economic performance, high safety in use and convenience in assembling and disassembling, and is a key device for assisting of completing launching after flat ground construction of the self-elevating platform.