Underground excavation transformation construction method for expanding excavation of new channel on basis of existing channel

The invention discloses an underground excavation transformation construction method for expanding excavation of a new channel on the basis of an existing channel. The underground excavation transformation construction method comprises the following steps that reinforcement treatment of the expanding excavation of the new channel is conducted through a bottom plate of the existing channel; a bottom plate breaking area at the vertical shaft within the construction vertical shaft range is broken; a construction vertical shaft is constructed downwards; a new channel is constructed through underground excavation; constructing a waterproof layer below the bottom plate of the new channel; constructing a newly-built channel bottom plate and a lower side wall below the existing channel bottom plate; breaking and removing a bottom plate of the existing channel; constructing a top plate and an upper side wall of the newly-built channel; and compactly filling plain concrete. Under the conditions that the surrounding env