In order to maintain a good indoor environment, fresh air is essential for audience in theatre buildings due to the high occupant density. Apart from mechanical fresh air, uncontrolled air leakage across building exterior (i.e., air infiltration) is also a source of outdoor fresh air. Therefore, assessing air infiltration rate is very necessary for determining total fresh air rate and avoiding excessive ventilation. However, the commonly used method to assess infiltration rate is very complex to implement. In this paper, we propose a convenient method to assess infiltration rate based on the mass balance of particulate matters. Given the ubiquity of particulate matters in the outdoor environment and being cost-free, this method is able to avoid the disadvantages of traditional methods. PM2.5 is chosen to establish the mass balance. Infiltration and total fresh air rates of a concert hall were measured and analyzed using the proposed method.