The ventilation rate of the reactor building is determined by the calculation results of radiation protection,which has referred to the demands of code,the experience of German FRMII reactor designing,isolation valves,of which the in-out-leakage rate is 0 under the reactor building tightness test pressure(12.5kPa),are set on the air ducts which through the sealing boundary of operating hall.This measure can guarantee the radioactive matters against leaking into outside spaces through the ventilation ducts during accident conditions of the reactor.Direct-connected steel fans and integral stainless steel air cleaning equipments are chosen in the system designing.