Transformation and construction method for existing cavity in underground excavation region

The invention discloses a transformation and construction method for an existing cavity in an underground excavation region. The transformation and construction method comprises the steps: detecting an existing tunnel, and determining the expanding excavation construction; determining a boundary line to be broken down according to the measurement result; distributing small duct grouting supports on an arched part, chiseling away the existing tunnel arch wall structure section by section; cutting away reinforcing steel bars of the existing tunnel structure, erecting a steel grating, arranging feet-lock anchor bolts and erecting temporary supports; spraying concrete on the newly erected steel grating; after the arch wall is constructed for 20 meters continuously, removing the temporary supports step by step, removing the inverted arch concrete layer on the existing tunnel at the schedule of less than 2 meters each time, and carrying out inverted arch part initial support to form closing; and finally carrying out waterproof construction, the inverted arch second lining concrete layer and inverted arch filling concrete layer construction, and constructing the arch wall concrete layer by using a template trolley. The transformation and construction method has the advantages that the existing civil air defense space is more effectively used, the excavated volume of earthwork is reduced, the initial supporting reinforcing steel bars are less used, moreover, the construction safety is ensured, the construction period is reduced, and the transformation and construction method is first provided in China.