Stray light suppression is a key challenge in the development of high-performance coronagraphs. Laboratory measurements, however, are often affected by environmental interferences such as collimator-induced stray light, indoor reflections, and dust scattering. This study develops a methodology to quantify these environmental stray-light components for the Spectral Imaging Coronagraph and its test environment. We establish an empirical relationship between the intensity of the stray light and the dust-covered area on the collimator objective, enabling the quantification of the effects of dust accumulation level on the stray light. In addition, an artificial light source system that simulates laboratory conditions is developed to evaluate the impact of baffle-reflected light. To suppress dust scattering in optical channels, a grouped sequence image processing technique is applied, with a high-suppression white-light coronagraph serving as a reference. Experimental results demonstrate that the proposed method effectively quantifies environmental stray light, enhances measurement accuracy, and provides vital support for the development and testing of space coronagraphs with advanced stray light suppression capabilities.