EPU100 at SSRF is an APPLE-II type variable elliptically polarized undulator. The magnet structure consists of four magnet arrays with standard Halbach-type. Two arrays at one diagonal can move longitudinally on the rails so that different magnetic field distributions are shaped to meet the requirements of varieties of polarization modes. With the longitudinal moving of the magnet arrays

the magnitude and the direction of the magnetic forces changed and the distortions of the backing beams both in vertical and horizontal directions will be different. In this paper we analyze and calculate the horizontal displacement of the beams in different gaps and phase shifts to create an equivalent beam mechanical model which is consistent with the measured deformations.