The invention relates to a geotechnical and underground engineering technology field, particularly to a box-raft pressure-bearing type foundation underpinning and construction reinforcing method for tunnels across pile foundations of a bridge above a river. The method of the invention comprises: reinforcing abutment foundation of both sides of the bridge, reinforcing the riverbed outside the cofferdam construction and abutment, strengthening bridge supports, excavating pits under cofferdam bridges, strengthening expanding plate-type foundations inside the pits and ground base under the plates, as well as shield tunneling and removing obstructive file base. The method can smoothly complete construction and guarantee smooth shield tunneling while ensuring normal passage of existing bridge structure, further can effectively reduce construction risk, accelerate construction process, reduce cost and reduce influence to environment to a maximal extent.