In the view of the problem that there is no high precision universal magnification measurement methods and apparatus at present, a high precision magnification measuring method for imaging system based on double fiber point-diffraction interferometer is proposed. The separation distance between two fibers in object plane and the separation distance between the fibers’ imaging points in image plane are measured with nanometer accuracy by analyzing the quantitative relationship among the separation distance between double point source, spatial location of CCD camera and the Zernike polynomial coefficients of the phase distribution of the point-diffraction interference field. And then, high precision magnification measurement is achieved. Simulation and experimental verification have been carried out to demonstrate the feasibility and stability of the proposed measuring method. The results show that the expanded uncertainty of the magnification measurement is 2.64×10 -6. The proposed method has the merits of high accuracy, high efficiency and high reliability. It can be used to measure the magnification of high accuracy imaging system with ultra-high precision, such as microscope objectives and lithographic projection lens.