In this paper the formation theory of the digital shearing speckle interference fringes pattern is analysed, and the exact ralationship between fringes brightness and parameters of the TV camera numerical aperture etc. is obtained. A better digital shearing speckle interference fringe pattprn, indicated by the theoretical analyses and experiments, can also be obtained by using a smaller numerical aperture and a He-Ne laser about 3 mW. The theory and results herein have a better guidance to the optimel parameters selection for making the digital shering speckle interferometric experiments.