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Moiré technique is often used to measure surface morphology and deformation fields. CCD moiré is a special kind of moiré and is produced when a digital camera is used to capture periodic grid structures, like gratings. Different from the ordinary moiré setups with two gratings, however, CCD moiré requires only one grating. But the formation mechanism is not fully understood and also, a high-quality CCD moiré pattern is hard to achieve. In this paper, the formation mechanism of a CCD moiré pattern, based on the imaging principle of a digital camera, is analyzed and a way of simulating the pattern is proposed. A universal period formula is also proposed and the validity of the simulation and formula is verified by experiments. The proposed model is shown to be an efficient guide for obtaining high-quality CCD moiré patterns.