Abstract: The undulator magnetic fiducialization is an important prerequisite to ensure the undulator segments' installation precision of the FEL device.A device called"magnetic landmark"was developed to achieve high precision undulator magnetic fiducialization.The landmark consists of several permanent magnets which can generate a normal and a skew quadrupole field with high gradient in two orthogonal directions.Positions of the zero point of the two quadrupole fields' horizontal component were measured and the specific use of the landmark is given.The results show that the accuracy of the magnetic center axis of an undulator after fiducialization in the landmark coordinate system is better than ±20μm in the horizontal direction and ±2μm in the vertical direction.