In order to obtain the generating law of background fringes in the video of a scanning CCD camera induced by repetitive pulse lasers, a mathematic model describing fringes induced by repetitive square pulse lasers is established. In this model, the relation of camera parameters including stage number of time delay integrations, integration time of one stage as well as laser pulse width and laser repetitive period can be used to estimate whether the fringes appear or not and to calculate what is the dimensions of fringes, in which the precision of calculation is 1 pixel. An experiment is performed by using repetitive picosecond pulse lasers and a fluorescent lamp to irradiate the scanning CCD camera respectively, and the results validate the correction of estimation and calculation based on the model. Finally, combing the intensity relation of background light and scattered laser with the model, the visibility expression of fringes is given, which is also validated in the experiment.