A non-synchronously-numerating experimental system is applied to measure the gain of two-wave coupling in a Ce∶KNSBN crystal while the signal beam and the pump beam are non-simultaneously opened, and the effect of fanning on the dynamical characteristics of two-wave coupling is discussed. It is shown that there is significant writing beam intensity threshold effect of fanning as 632.8 nm He-Ne laser is used to write the volume grating in Ce∶KNSBN crystal, and obvious fanning occurs only if the writing beam intensity is higher than 30 mW/cm2. Modified wave-coupling equations are used to simulate both the dynamical process of two-wave coupling and the gain as a function of the opening time interval between the signal beam and the pump beam. The theoretically calculated results are coincident with the experiment data.