The utility model discloses a magnetic field detector for an electromagnetic stirrer. The magnetic field detector comprises a main body straight-bar, a bearing, a velocity measurement wind wing plate and a plurality of magnetic field cutting rods, wherein the inner ring of the bearing is sleeved and fixed on the main body straight-bar; the velocity measurement wind wing plate is fixed at a first end of the main body straight-bar; each magnetic field cutting rod is fixed at a second end of the main body straight-bar; the axis of the main body straight-bar is parallel to or on the plate surface of the velocity measurement wind wing plate; the axis of each magnetic field cutting rod is parallel to or on the plate surface of the velocity measurement wind wing plate; the bearing is close to the first end. By adopting the magnetic field detector for the electromagnetic stirrer, disclosed by the utility model, magnetic field intensity can be reflected by the rotating speed of the velocity measurement wind wing plate, so that a user can conveniently test whether electromagnetic stirring parameters of the electromagnetic stirrer are reasonable, and can favorably adjust the electromagnetic stirring parameters in time according to test results, and the quality of casting blank finished products is further ensured.