Dynamic Range is an important index to evaluate the performance of scientific CCD imaging system, which reflects the detection capability to the signal. At present, the description of DR is mainly expressed by the ratio of CCD full well capacity to readout noise at home and abroad. On the basis of clarifying the imaging process of scientific CCD, a method that defines linearity critical point as the last point where the Linearity Error doesn't exceed -3%, minimum exposure point as the point where the SNR equals to 1, then precisely expresses and calculates DR is introduced. Afterwards in the lab, 100 light and 100 dark images of CCD at different exposure time are collected by using the integrating sphere and other equipment, the relationship between the signal and exposure time is calculated to find out the linearity critical point and minimum exposure point and calculate the DR of CCD. The result shows that the DR of the measured CCD is 61.76dB, which corresponds to the theoretical analysis and verifies the validity of the test. Finally, the factors influencing the DR of CCD are analyzed, the advice to improve the DR is introduced.