This paper introduces a polarization state generator into an imaging polarimeter to calibrate the instrument and proposes a double-calibration method for the imaging polarimeter. The standard polarization states generated by the polarization state generator are measured by the polarimeter to obtain the polarimeter response matrix and thereby to calibrate the potential errors in the polarimeter for the first time. The errors of the fast-axis azimuth angle and the linear dichroism coefficient of the quarter-wave plate in the polarization state generator can then be solved with the optimization algorithm to achieve a double-calibration of the standard polarization states and the polarimeter response matrix. The method is tested in the laboratory, and the errors in the polarization state generator and the calibrated polarimeter response matrix are solved. Double-calibration of the imaging polarimeter is thus achieved. The experimental result shows that the method can improve the measurement accuracy of the imaging polarimeter.