Purpose / Context - During the process of ventilation, outdoor fine particulate matter enters indoor space through gaps in the external window of building and pollutes the indoor environment. Penetration factor (P), deposition loss rate (k), and air exchange rate (a) are important parameters to evaluate the number of outdoor fine particles infiltrate into indoor space and the exposed quantity of indoor personal fine particulate matter. At present, these parameters are mainly obtained through the methods of laboratory actual measurement or theoretical derivation. Methodology / Approach - In this study, according to the law of indoor-outdoor particle mass bal-ance and statistical theory, a novel method for estimating the above three parameters was developed, which dependent on a large number of indoor and outdoor PM2.5 mass concentrations field moni-tored data. Results –The results of the method application in three typical office buildings showed that the value of penetration factor (P) was influenced by the external window air-tightness level obviously, it was about 0.965 when external window air-tightness in level-4 and it was 0.920 when external window air-tightness The influence factors of penetration factor (P) and deposition loss rate (k) are different by analyzing in the last section Key Findings / Implications –The value of P, k can be treated as be as a fixed value. The window with different structure has different value of P. Originality - A new method of Reference can be provided to study the windows crack permeability, predict the impact outdoor PM2.5 on indoor environmental and analyze the PM2.5 exposure of indoor.