The invention provides a method for detecting the welding deformation amount of a vane, which does not need to use any expensive detecting instrument, has simple process and achieves a reliable and visualized detection result, and is suitable for detecting large-size vanes on a large scale, wherein the method comprises the following detection steps of: (1) positioning the vane by three central holes before welding; (2) detecting the distance between a test point on each molded surface of each tap position of the vane and a pallet; (3) integrally welding the vane with a cover plate and detecting the distance between each test point of the vane and the pallet on each tap position by using a filler gauge; (4) detecting the distance between a clamping leg of the pallet on each tap position and a measuring frame; (5) solving a measured value of the distance between each test point and a theoretical envelope curve of the vane after welding the vane; and (6) correspondingly subtracting the measured value before welding the vane from the value of the distance between each test point and the theoretical envelope curve of the vane after welding the vane and obtaining a difference value thatreflects the welding deformation amount of a cross section for the pallet of the vane.