Through proposing a set of differential eguations governing the harm gas concentration distribution in return airways of the recirculating region and in combination with calculated ventilation network, non-steady state of recirculating ventilation bas been studied in more detail. 通过建立循环通风系统循环区回风道有害气体浓度微分方程,并结合网络解算理论,对循环通风系统非定常情况进行了细微研究。