Construction method for tunnel passing through existing road under small clear distance

The invention relates to the technical field of tunnel construction, in particular to a construction method for a tunnel passing through an existing road under a small clear distance. The small-clear-distance underneath pass existing road tunnel adopts a double advanced pre-support system of an advanced large pipe shed and advanced small pipe grouting; before a small-clear-distance underneath pass existing road tunnel is excavated, a pipe shed working cavity is excavated at a certain distance behind the small-clear-distance underneath pass existing road tunnel in the tunneling direction, a pipe shed working cavity supporting structure is constructed, and the pipe shed working cavity is used for constructing an advanced large pipe shed; the advanced large pipe shed construction specifically comprises the steps that S0, the design direction of a large pipe shed is determined; s1, guide wall construction; s2, a guide frame is installed; s3, drilling is conducted in the circumferential direction of the guide wall