The invention discloses an orientation distributing device and method for ventilation volume of tunnel construction. The device comprises a distributing box and a mounting support for horizontally mounting the distributing box; the distributing box comprises a distributing box body of which the front part and the rear part are opened, an air outlet structure arranged at the front side of the distributing box body and an air inlet structure arranged at the rear side of the distributing box body; the air inlet structure comprises left and right air inlet joints which are mounted together; the air outlet structure comprises left and right air outlet joints which are mounted together; the inside of the distributing box body is divided into left and right air supply channels by a separating device; and the separating device is vertically arranged and comprises a separating seat fixed in the distributing box body, a movable door and a swing driving device. The method comprises the following steps: firstly, mounting a ventilation system of a tunnel; and secondly, performing orientation distribution of the ventilation volume. The orientation distributing device disclosed by the invention is reasonable in design, simple and convenient to use and operate and good in service effect; and the ventilation volumes of two tunnel holes in the construction process can be flexibly distributed simply and conveniently, and the utilization rate of ventilation equipment is improved.