Abstract: Experimental results of SGIII prototype facility show that dynamic distortion exists in streak camera. Simulations indicate that, if input light intensity becomes too large, space charge effect in image converter tube shall affect the focus of the electron beam and decrease the magnification, which result in dynamic distortion. With increase of current intensity, the dynamic distortion becomes more evident. All above assist us better understand the electric-optic system of image converter tube, and provide ideas for further experimental diagnostic design.