System gain is one of the important parameters of charge coupled device (CCD), from which many of the other performance parameters such as quantum efficiency, readout noise, full well CCD and so on are determined. With the development of imaging technology, accurate calibration for the system gain of CCD become more and more important. The theory of measuring system gain based on photon transfer curve technique is introduced. A method to acquire photon transfer curve by changing light intensity is proposed. At the same time, an experimental setup is constructed to calibrate system gain of CCD precisely. It is shown that the result of calibration is 1.6229e-/ADU which has a good consistency with the method to obtain photon transfer curve by changing exposure time. At last, the uncertainty of this calibration is evaluated.