The invention discloses a bottom longitudinal beam construction method for subway construction. The method comprises the following steps that (1), a waterproof layer of a bottom plate is laid; (2), aprotective layer is laid on the waterproof layer; (3), a bottom longitudinal beam and a part of the bottom plate are connected through steel bars; (4), a template is mounted at the edge of the bottomlongitudinal beam; (5), concrete is poured after the template is installed; and (6), curing is carried out. The construction method has the beneficial effects that the bottom longitudinal beam is constructed according to a series of measures, the measures include that the laying of the bottom plate waterproof layer, the laying of the 7 cm protective layer, the connection between the bottom longitudinal beam and a part of bottom plate through the steel bars, template installation, concrete pouring and curing, the mounting strength of the bottom longitudinal beam is effectively improved, and theservice life is prolonged.