During the operation of high-energy proton accelerators, the air in the tunnel is activated with the production of radionuclides. For CSNS (China Spallation Neutron Source), the first pulsed neutron source in China for multidisciplinary research, an online air activation monitoring system was developed to evaluate the radiation safety of the staff and the public, which consisted of a NaI detector, Pb shielding, an MB container and a control system. With the monitoring system, gamma spectra of the activated air from controlled areas are measured, and the activity concentration and immersion dose rates of radionuclides in air are calculated and displayed in real time. The system has been in stable operation since February 2020, and results have been obtained for the evaluation of the radiation risk from activated air.