The passive Differential Optical Absorption Spectroscopy (DOAS) using sun scattered light can perform observation of local total volume of trace gases. To reduce the effect of Ring effect on passive DOAS system, the mechanisms of Ring effect were analyzed in this paper, and the scattered light was divided into elasticity and non-elasticity scattering. The expressions of Ring effect are educed. The calculated Ring is retrieved together with other atmospheric components. Therefore, its effect on the measurement could be restricted to minimum in DOAS system. The experimental result shows that the accuracy of DOAS system is improved.