The modulation of photon polarization by liquid crystal variable retarder (LCVR) whose phase retardation can be controlled easily by its driving voltage is described. A spinning polarimeter is designed to calibrate the LCVR for 820nm wavelength in situ. The driving voltages corresponding to 1/4λ,3/4λ,1/ 2λ and 1λ of LCVR are obtained,and the polarization of photon is modulated by changing the driving voltage via a computer. The theoretical analysis and experimental result are presented. The advantages of the proposed method are insensitivity to the incident photon direction,large wavelength range without mechanical movement,and real-time control.