The invention discloses a soft rock large-deformation tunnel supporting system and a construction method thereof. The system comprises a forepoling, a preliminary bracing, a waterproof layer, a secondary lining and other main structures, wherein the structures are constructed in sequence according to construction procedures. Surrounding rock is reinforced through advanced small pipe grouting by means of the forepoling, the preliminary bracing comprises lengthened anchor rods, a primary spraying layer, steel arches and a re-jetting layer, feet-lock bolts are further arranged at the positions of an upper bench and arch feet to be welded to the steel arches, the adjacent steel arches are connected through longitudinal connecting ribs, and the secondary lining is constructed at last to form the supporting system. The steps are simple and linked with one another, supporting lag caused by idling of labor force is avoided, and large-deformation of tunnels is well controlled.