A two-wave coupling experimental system is used to study the beam fanning effect in a Ce:KNSBN crystal under a non-synchronous readout condition. The influence of the incident beam polarization state, intensity and incident angle on the beam fanning effect is studied with single beam incidence. The results show that when the beam has extraordinary polarization, the beam fanning effect is more evident and there exists an obvious intensity threshold of 38.2 mW/cm2. For a constant incident angle, the steady beam fanning intensity increases with the incident beam intensity. For a constant incident beam intensity, the steady beam fanning intensity increases with the incident angle to a peak value with 15° incident angle, and then declines with the rise of incident angle. The incident beam intensity threshold characteristic and the influence of the incident beam polarization state, intensity and incident angle on the steady beam fanning intensity are physically explained.