We establish a polarimetric reference for the degree of linear polarization (DOLP) measurement calibration, based on direct and reflected solar light, with a theoretical error of about 0.0012. This calibration source can be used to calibrate polarized radiometers instead of complex laboratory devices and can respond from UV to near infrared wavelengths. A two-step method for calibrating the DOLP measurement is proposed and applied to a ground-based polarized radiometer. The first step is correcting the transmittance difference between polarizer units using the direct solar beam, while the second step corrects possible bias in DOLP measurement using the reflected solar light as a reference. Based on instrument characterization, calibration results obtained with the new polarized sun-sky radiometer, CE-318-DP, are discussed and compared with laboratory results.