The invention discloses a three-step inverted arch synchronous excavation and support construction method. During tunnel construction, an upper step (1), a middle step (2), and a lower step (3) synchronously tunneling and synchronously supporting, and the lower step (3) excavation comprises an inverted arch (4); the construction comprises the following steps that excavating is carried out, specifically, three-step synchronous drilling, synchronous charging and line connecting are carried out, and detonating is sequentially carried out according to the upper, middle and lower sequences; supporting is carried out, specifically, deslagging and supporting are carried out according to the upper, middle and lower sequences; spraying is carried out, specifically, concrete spraying is sequentiallycarried out from bottom to top; in the excavating and supporting steps, a long-arm excavator is adopted to directly lift and transfer a tool and a material on a backfill hole slag (7) to the upper step (1); and in the spraying step, a long-arm wet-spraying manipulator is adopted to directly spray concrete on the upper step (1) on the backfill hole slag (7). According to the three-step inverted arch synchronous excavation and support construction method, the operation advantages of the long-arm construction machines are fully utilized, the procedure of building a transport channel is simplified between steps, the construction progress is obviously accelerated, and the three-step inverted arch synchronous excavation and support construction method is suitable for the construction of a softsurrounding rock tunnel three-step method.