The invention relates to the technical field of tunnel construction, in particular to a two-unit and six-step construction method for a shallow-buried large-span tunnel. The two-unit and six-step construction method solves the problem that a construction method which is simple in technology, can achieve the effects of isolation and supporting in the tunnel construction process is lacked in existing soft soil tunnel construction in a water-saturated area. The two-unit and six-step construction method includes the steps of excavating an upper bench of an upper unit and constructing preliminary supporting; excavating an lower bench of the upper unit and constructing preliminary supporting; mounting a cross beam at the separation position of an upper unit body and a lower unit body and constructing a concrete center septum; mounting a support arching on the center septum; excavating an upper bench of an lower unit and constructing preliminary supporting; and excavating and constructing an inverted arch and filling of the inverted arch, and completing enclosing of a tunnel face and forming a ring. By adoption of the two-unit and six-step construction method, water stored in a reservoir is effectively prevented from seepage, not only is the surrounding environment of the reservoir is protected, but also the construction safety is ensured, special equipment is not needed to be invested in construction, the construction quality meets the structural requirements, and the construction method is convenient to operate, and has strong generalization performance.