The invention discloses a construction method for a railway under a tunnel. The construction method is characterized in that dewatering deep wells are arranged; water at a water-rich drift sand layer section of an excavated tunnel is introduced into the dewatering deep wells, and is extracted out of the dewatering deep wells, so that pressure-bearing underground water in a construction area is drained, and therefore, the water level and depth in the center of a construction area are effectively controlled to achieve construction requirements under a water-free environment, phenomena of sand drifting and piping are avoided, and the stability of an excavation face is ensured. Besides, according to the method, cover plates and retaining walls are arranged at the two sides of a railway subgrade, and side slopes between the retaining walls and the railway subgrade are filled with concrete, so that cover plate sections are formed at the two sides of the railway subgrade; and tunnel extraction is divided into open excavation, cover excavation and underground excavation, so that the underground excavation length is reduced, and therefore, the construction risks are reduced.