The typical optical system of scatter plate interferometry is strictly analysed by means of modern optics. The equivelence of two beams, one of which is scattered first and transmitted secondly and the other is tromsmitted first and then scattered, is proved by Fourier optics. The variation of the two optical pathes caused by wave aberration of tested lens is derived. Forthermore, the fringe mechanism, its contrast, the relationship between the correlation of transmittence of two plates and the contrast are discussed by statistical method. The fringe formula of the interferometry is established. The analysis coincides with the experimental results and can explain some phenomena of scatter plate interferometry that could not be explained before.