We propose a fast calibration method to compensate the non-uniform illumination in computational ghost imaging. Inspired by a similar procedure to calibrate pixel response differences for detector arrays in conventional digital cameras, the proposed method acquires one image of an all-white paper to determine the non-uniformity of the illumination and uses the information to calibrate any further reconstructed images under the same illumination. The numerical and experimental results are in a good agreement, and the experimental results showed that the root mean square error of the reconstructed image was reduced by 79.94% after the calibration.