The invention belongs to the technical field of civil engineering support, and particularly relates to a pre-support method for a tunnel having undesirable stratums, particularly water-rich stratums and a special tool for the method. The method mainly comprises the following steps of: 1.1 drilling, namely drilling along the designed periphery of the tunnel at intervals of 0.3 to 1.0m during construction, wherein the drilled holes are horizontal or in an inclination angle below 10 degrees; 1.2 inserting a longitudinal opening perforated pipe into each of the drilled holes and making the bottomof the longitudinal opening perforated pipe adjacent to the designed excavating boundary of the tunnel; 1.3 placing a high-pressure jet-grouting nozzle into one of the drilled holes to perform high-pressure jet-grouting operation; and 1.4, performing jet-grouting to form a plurality of jet-grouting bodies, wherein a pre-support arched shed structure is formed by the plurality of jet-grouting bodies and the longitudinal opening perforated pipe, and the inner liners of the tunnel, the longitudinal opening perforated pipe and the jet-grouting bodies are tightly combined to form an effective support structure. The invention also discloses the special tool for the method.