With the vigorous development of building fresh air filtration systems, the determination of outdoor design PM_2.5 concentration has become one of the important factors for the selection of fresh air filtration systems. In this paper, a simple and effective method was proposed to estimate the outdoor design PM_2.5 concentration, through which the outdoor design PM_2.5 concentration for any location of China can be quickly calculated by using the recommended coefficient K and annual average PM_2.5 concentration of the provincial capital city. The recommended coefficient K should be given based on the specific regions. The different coefficient K determination methods were put forward based on several existing region divisions. The hourly average outdoor PM_2.5 concentrations in 31 cities of China in 2017 were collected. The daily average values, the annual average values and the corresponding values of different regions under different non-guarantee rates were calculated. Taking several typical cities as examples, the effect of different recommended coefficients K on the selection of air filters in fresh air filtration systems was compared and analyzed. The results showed that the mathematical inductions method was more suitable for estimating the outdoor design PM_2.5 concentration at this stage. The recommended coefficient K should be divided into strict conditions and normal conditions in use. It is recommended to use the recommended K of geographical region for the air filter selection. This paper would help the designers and researchers engaged in relevant research to select the suitable air filter of the building fresh air systems fast.