With different X-ray tube voltages the imaging screening effect of the pixellated CdZnTe detector is investigated under the ultrahigh irradiance. Moreover, a novel fluctuation process of the screening effect has been observed. Measurement results reveal that event counts of pixels which are distributed at the edge of the irradiated area experience a sudden increase followed by a continuous decrease when the tube voltage rises from 15 kV to 45 kV. Based on the Poisson equation, a theoretical model of the CdZnTe detector solved by the finite element method is developed which enables the penetrating investigation of the carrier collection in the CdZnTe crystal. A comparison between model simulations and test results shows that pixels in the central irradiated area are completely screened under increasing incident photon energy and linearity attenuation parameter. This is because the emergence of the relatively higher potential region, which is caused by the increasing of incident photon energy and linearity attenuation parameter, leads to the twist of the electron drift way. Furthermore, with the increase of the incident photon energy, the inconsecutive expansion of the accumulated carrier scope results in the fluctuation of the event counts of pixels distributed around the screening area. The imaging results deduced from the simulations are well consistent with the experimental data.