The invention discloses a soft rock tunnel construction method on the basis of flexible formwork support. A soft rock tunnel is divided into a plurality of segments from back to front to be subjected to construction. Construction of any one segment comprises the following steps that (1), basic mechanical parameters of surrounding rocks are defined; (2), the tunnel reserved excavating quantity is defined; (3), the tunnel begins to be excavated; (4), A tunnel support structure is defined: the tunnel support structure comprises a tunnel incipient support structure, a flexible formwork support structure and tunnel two-time linings; (5), tunnel incipient support is constructed; (6), the flexible formwork support is constructed; (7), the tunnel two-time linings are constructed; (8), a next segment is constructed; and (9), step (8) is repeated for many times until all the construction processes of the soft rock tunnel are completed. According to the soft rock tunnel construction method on the basis of the flexible formwork support, the method and the steps are easy, design is reasonable, achieving is convenient, and the using effect is good; and the reserved excavating quantity is defined according to deformation conditions of the surrounding rocks of the soft rock tunnel, the flexible formwork support structure is adopted to support full cross sections of the tunnel and subjected to coordination deformation with the tunnel incipient support structure, and thus structure stability of the tunnel two-time linings is guaranteed.