Polarimeters used in cosmic microwave background (CMB) experiments must be well calibrated to measure faint CMB polarization patterns with low systematic errors. Polarimeter characteristics generally vary with the incident load temperature (Tload). Therefore, re-producing the observing conditions in the laboratory is an important concern. For polarimeters, we developed a characterization system with cryogenically cooled loads. The loads generate unpolarized radiation (15 K and 30 K), comparable to the typical sky temperature of the best sites on the ground, e.g., the Atacama Desert in Chile (T load ∼ 15 K). The radiation from the loads is reflected by a metal mirror in the cryostat, yielding partially polarized radiation (600 mK), entering a feed horn on the polarimeter. Rotation of the mirror alters the incident angle of the polarization and causes periodic switching of the load temperature for Y-factor measurements. We demonstrated the validity of the system using a polarimeter developed for an upgrade of QUIET (QUIET-II), which can obtain the Stokes parameters I, Q, and Usimultaneously. The system characterized all the necessary properties, e.g., the responses for I, Q, and U, and their crosstalk. In addition, a wide range of polarimeter bias conditions was surveyed. The principle of the characterization system is not limited to a particular frequency or detection scheme. Thus, various types of state-of-the-art detectors can be calibrated by using this system.