Humans spend most of their time in indoor environments, thus a thorough understanding of indoor and outdoor PM_2.5-bound polycyclic aromatic hydrocarbons (PAHs) origins for accurate assessment of health risks is required. In the present study, 84 pairs of PM_2.5 samples from indoor (laboratory) and outdoor (campus) locations were collected from April to December 2018 in Caofeidian, China. The annual median concentration of PM_2.5 outdoors was 90.80 µg/m³, 9.08 times higher than the annual standard of WHO guideline (10 µg/m³). Indoor PM_2.5 annual median concentration (41.80 µg/m³) was also higher than the annual standard of ASHRAE guideline (15 µg/m³). The annual median concentrations of ∑₁₈PAHs indoors (44.23 ng/m³) and outdoors (189.6 ng/m³) were highest in winter and descended in the order of autumn > spring > summer. Contrary to summer and autumn, indoor/outdoor concentration ratios were less than 1 in spring and winter, indicating that the contribution of outdoor particle infiltration was more significant than that of indoor sources. The positive matrix factorization model suggested that indoor PAHs came from three sources: vehicle emissions (43%), biomass burning (37%), industry emissions, and coal combustion (20%). Outdoor PAHs came from four sources: petroleum volatilization (39%), vehicle emissions (30%), coal combustion (18%), and biomass burning (13%). The incremental lifetime cancer risk values of indoor and outdoor PAHs in winter exceeded the acceptable level (10⁻⁶), and the carcinogenic risk of adults was higher than that of children and teenagers. These results indicated that simultaneous monitoring of indoor and outdoor PAHs is recommended for accurate assessment of health risk, and the analysis in the current work should be helpful to formulate policies to reduce PAHs emissions.