Abstract: The long horizontal and vertical sampling pipelines are used in the gaseous radioactive ef-small corrective room at the bottom of the stack,therefore,85.4 meters long horizontal sampling pipe is can-celled,only 70.6 meter long vertical sampling pipe is reserved.The test and assessment result indicated that the corrective factor of the radioactive iodine sampling is 1.65.For radioactive particulate,according to con-servative estimate,the corrective factor is less than 1.65 under accident condition.During normal operation,fluent monitoring system in Qinshan Nuclear Power Plant,so there is design deficiency for the monitoring of gaseous effluent.The long sampling pipelines,especially horizontal pipe,will result in the losing of sampling particulate and iodine.By means of modification,the sampler for particulate and iodine is moved into the the particulate is consisted of tiny particles,and the corrective factor is 1.1.