Most city trees in Nanjing are platanus acerifolia and populus nigra, which generates lots of whirling willow catkins in the air in the transition season. However, enough attention has not been paid to the health risk for occupants in roadside buildings where human skin itching and respiratory infection may be incurred. It was found that the indoor air quality cannot meet WHO guideline requirement in transition season due to the influence of plant pollutants. It is urgent to find a suitable ventilation scheme, together with air filtration measures, to improve indoor air quality for roadside office in transition season. Four ventilation schemes were compared, including natural ventilation, no ventilation with door and windows closed, recirculating ventilation with operation of ceiling cassette fan-coil unit without air filter, and recirculating ventilation with operation of ceiling cassette fan-coil unit with F7 filter installed. The performances of ventilation modes were evaluated by comparison of the impact of outdoor particulate matter on indoor air quality. Results showed that the larger the particle size was, the lower the I/O ratio was. The production of indoor particles by occupants’ activities cannot be ignored. The influence of occupants’ activities on large particles was greater than that on small particles. It is suggested to operate the ceiling cassette fan-coil unit with air filter for this application, which can reduce the indoor particle concentration to an acceptable level and decrease the potential health risk posed by plant pollutants.