Abstract: In order to evaluate the actual purification effect of air purifiers in primary school classrooms and dormitories, a cross-intervention study was carried out in a boarding primary school in Beijing from March to April 2019. Classes for grades 4 to 5 were selected and divided into 2 groups. The air purifiers equipped with high efficiency particle air filter (HEPA) were placed in one group of classrooms and dormitories, while the air purifiers without HEPA were placed in the other group. After 4 days of intervention and a one-month washout period, the air purifiers were exchanged for the second intervention. The indoor and outdoor concentrations of PM_2.5, PM₁₀ and PM₁ were monitored in accordance with students' schedules, and the purification rates of the air purifiers were calculated based on the intervention group and non-intervention group. A multiple linear regression model was used to analyze the influence of outdoor PM_2.5 concentration, intervention, personnel activities, and ventilation on indoor PM_2.5, and to analyze the factors influencing the purification effect. The results showed that: (1) The purification rates of PM_2.5, PM₁₀ and PM₁ were 41.3% (lower quartile (Q_l)-upper quartile (Q_u): 0-53.1%), 40.7% (10.5%-46.2%), and 34.9% (9.6%-40.3%), respectively. PM_2.5 had the highest purification rate. The purification rates of PM_2.5 in dormitories were higher than that in classrooms, and similar findings were found for PM₁₀ and PM₁. (2) When the outdoor PM_2.5 concentrations was in the 115-150 μg/m³ group, the highest purification rate of the air purifier was 52.83% (50.26%-56.13%), and similar findings were found for PM₁₀ and PM₁. (3) Multiple linear regression analysis determined that when outdoor PM_2.5 concentration < 35 μg/m³, the indoor PM_2.5 concentration decreased by 3.73 μg/m³ (95% confidence interval (95%CI): (0.60 μg/m³, 6.86 μg/m³)) with open-door ventilation, and indoor PM_2.5 concentration increased by 3.4 μg/m³ (0.22 μg/m³, 6.58 μg/m³) in the case of indoor personnel activities. When the outdoor PM_2.5 concentration was between 35 μg/m³ and 150 μg/m³, the indoor PM_2.5 concentration was reduced by 33.36 μg/m³ (16.47 μg/m³, 50.25 μg/m³) with HEPA air purifier. When outdoor PM_2.5 concentration ≥150 μg/m³, the indoor PM_2.5 concentration was reduced by 48.87 μg/m³ (25.62 μg/m³, 72.12 μg/m³) with HEPA air purifier, while the indoor PM_2.5 concentration was increased by 37.65 μg/m³ (5.60 μg/m³, 69.69 μg/m³) with open-door ventilation. This study found that the use of air purifiers can not only reduce the concentration of indoor PM_2.5, but also reduce the concentration of PM₁₀ and PM₁ in the normal life and study of primary school students. When the outdoor PM_2.5 concentration is lower than 35 μg/m³, there is no need to turn on the air purifier. When the outdoor PM_2.5 concentration is in the range of 35 μg/m³ to 150 μg/m³, the air purifier has a good purification effect, and occasionally opening windows for ventilation does not affect the purification effect of the air purifier. In the case of a large number of students in the classroom, if the air purifier is turned on, proper ventilation can be used to maintain a better indoor air quality. When the outdoor PM_2.5 concentration is higher than 150 μg/m³, the doors and windows should be closed if the air purifier is turned on to avoid affecting the purification effect of the air purifier.