The transient distortion of a CCD under laser irradiation is explored. A HS-41-02K30 camera with a linear CCD image sensor is irradiated by a 532 nm CW laser, and the experiment shows that a laser spot and three dark spots with the similar size and shape appear in the direction of photosensitive array with an equal space synchronously. These dark spots are a transient distortion phenomenon of the CCD camera under laser irradiation, and are named the shadows of laser spot in this article. The regularity and generation mechanism of the shadow phenomenon are analyzed. It suggests that the inner mechanism of generating shadows is the interaction between four parallel readout circuits of CCD which share the same bias. The expression describing the interaction is deduced, which indicates that reducing the resistance between the bias source and the common node of four readout circuits can weaken the shadow phenomenon, and using four voltage sources to provide bias for four readout circuits respectively can eliminate the shadow phenomenon. Obtained results point out that the expression deduced by this paper not only can explain the shadow phenomenon but also can provide references for the design of CCD in weakening and eliminating the shadow phenomenon.