The surfaces of flexible printed circuit products are discontinuous, it is imperative to provide a non-contact measurement system to measure not only non-continuous surface, but also surface topography and small dimension. Optical detection system based on differential astigmatic can solve the problem. In the existing literature, the mathematic model of differential astigmatic optical system is established by Geometrical optics, it brings imprecision factors. So it is necessary to establish a precise mathematical model of differential astigmatic. This paper analyzes the shortcomings and inadequate of the mathematic model of differential astigmatic optical system which is based on geometrical optics of traditional optical systems, and then analyzes the model based on laser Gaussian beam model, which is more accurately closer to actual situation. Subsequently, according to the mathematic model, this paper optimizes the optical path parameters and also analyzes the impacts of position errors of optical components upon the measurement, which lays the foundation of the improvement of the online FPC quality control system.