This study validated the chemicals monitoring ability of a polar organic chemical integrative sampler (POCIS) under non-steady-state conditions resulting from natural disasters and environmental accidents in aquatic environments via laboratory experiments. The goal of this work was to contribute toward monitoring the actual state of chemical contamination resulting from emergencies and natural disasters. A chamber replicating the chemical contamination of an aquatic environment established by a chemical exposure assessment model was set up in a clean booth. The concentrations of seven neonicotinoid pesticides were increased to a maximum of 1,000, 100, or 10 µg/L for one day and decreased by 50% per day thereafter. A POCIS was set up with polyethersulfone as the permeation membrane and a resin (Oasis hydrophilic-lipophilic balance) as the receiving phase. Calibration tests of the POCIS were also conducted. The results of this study led to the addition of six neonicotinoid pesticides to the list of POCIS-measurable targets. The results of the chamber experiments showed that a POCIS is a good tool for chemicals monitoring under non-steady-state conditions in an aquatic environment.