Based on the partial coherence theory, this paper derives the formula of intensity and variance of diffraction from a screen with randomly- distributed holes. The diffraction characteristics of the screen are analysed in detail, and the effect of spatial characteristics of the source on diffractive image is discussed. It has been shown that the light intensity distribution of diffraction field depends on geometric characteristics of the holes and the screen, and shape and size of the light source as well. This paper also shows that under certain illumination, both average intensity and variance distributions in diffraction field will not change if the number or size of definitely-shaped holes in the screen randomly changes.