The invention provides a construction method for opening section steel grouting to reinforce a tunnel. The method comprises the following steps: (1) firstly, measuring tunnel deformation conditions, according to a measurement result, manufacturing a plurality of opening section steel units, the shapes of the opening section steel units being matched with a to-be-reinforced intrados of a shield segment; (2) installing the opening section steel on the to-be-reinforced intrados of the shield segment; fixing and connecting the plurality of opening section steel units in series in sequence to form the opening section steel, a hollow circular arc pipe channel being formed in the opening section steel; (3) providing a plurality of fixing connecting pieces to respectively fix the opening section steel units on the to-be-reinforced intrados of the shield segment; (4) and arranging filling holes on the opening section steel, and injecting filler through the filling holes. The method is characterized by good reinforcing effect, low cost, short construction period, and high shape forming property in reinforcing processing on an original tunnel structure.