A tool for detecting included angle between the inner inclined plane of a shoulder and a rail bearing surface of a CRTS III type rail slab steel mold comprises a clamping plate used for fixing a universal angle ruler and a handwheel screw rod; the front part of the clamping plate is wedged, and the wedged surface is propped against a bulge on the edge of a rail cant; a universal angle ruler fixing groove is formed on the top surface of the clamping plate along the middle line direction; at least two handwheel screw rod mounting holes communicated with the universal angle ruler fixing groove are formed on the same side of the clamping plate; the handwheel screw rod passes through the hand wheel screw rod mounting holes to be fixed with the universal angle ruler arranged in the universal angle ruler fixing groove; and the bottom surface of the clamping plate lies on the rail bearing surface of the steel mold. The utility model has the advantages that the included angle between the rail cant of the rail slab and the inner inclined plane of the shoulder of the CRTS III type rail slab steel mold can be converted to a certain degree so as to be capable of being measured through the universal angle ruler, further coherent detection can be accomplished; therefore, on one hand, the precision is ensured, and on the other hand, the efficiency is improved. The tool can be widely applied to the filed of rail slab steel mold detection of lines for passenger transportation.