A method for the determination of Dechlorane 602 (Dec 602),Dechlorane 603 (Dec 603),Dechlorane 604 (Dec 604) and Dechlorane Plus (DP) in wastewater by gas chromatography-negative ion chemical ionization mass spectrometry (GC-NCI/MS) has been developed. The water sample was extracted by classical liquid-liquid extraction method to collect the Dechloranes from both particle phase and water phase, cleaned up using silica gel and then analyzed by GC-NCI/MS under the optimized conditions of 150 ℃source temperature,150 ℃ quadrupole temperature,280 ℃ initial temperature and 1 kV electron multiplier (EM) offset voltage. The quantitation limits of Dec 602,Dec 603,Dec 604 and DP were 0. 01-0. 1 ng·L-1. The correlation coefficients were above 0. 99 for the calibration curves. The validity of this method was demonstrated by determination of Dechloranes in wastewater of a municipal wastewater treatment plant in Harbin,China. The results indicate that the recovery rates of all Dechloranes in wastewater are 74%-122% ,the relative standard deviations (DRS) are less than 13. 2% . The concentrations of Dechloranes in influent and effluent are 3. 2 ng·L-1and 0. 19 ng·L-1,respectively. Dechloranes are dominanted by DP in wastewater.