A reflection-mode controllable partially polarized infrared light source was designed to calibrate infrared imaging polarimeters. Theoretical calculation and measurement were carried out to verify the results. First, a theoretical calculation method was developed to calculate the polarization states of the output light based on the Mueller matrix of the aluminum coated flat mirror. Calculation results show that the output light is partially polarized with the degree of linear polarization (DoLP) smaller than 0.80 and the degree of circular polarization (DoCP) smaller than 0.60. Then a long wavelength infrared (LWIR) imaging polarimeter was employed to measure the real polarization states of the output light. Experimental results show that the output light is partially polarized with the DoLP within a range of 0.25 and 0.85 in LWIR. The relative standard deviation of DoLP is less than 6% when the incident angle for the flat mirror is less than 80°.