The Shanghai Soft X-ray Free-Electron Laser (SXFEL) Facility is the first coherent X-ray light source in China with a wavelength covering the water window. An elliptically polarized undulator (EPU) will be installed to meet the needs of different users, so that the output laser can be switched between linear polarization and circular polarization. This paper presents the polarization control design for the user facility, the evaluation of bunch energy jitter, and analysis of radiation power stability. Moreover, a switching structure of the circular polarization direction is also designed, so the polarization of the output laser can be switched between opposite directions on the base of permanent magnet oscillator at the rate of tens of Hz.