Based on GC-MS/MS， a stable isotope dilution method was established for the determination of dioxins and dioxin like polychlorinated biphenyls in fish. The sample was extracted by rapid solvent extractor for 3 times， and the fat was removed by acidified silica gel. Then， after purification by multi-layer silica gel column， alkaline alumina column and activated carbon column， the elution samples containing PCDD/Fs and DL-PCBs were collected respectively. Finally， it is concentrated and redissolved for GC-MS/MS determination. In this experiment， the extraction， purification and determination steps of samples were optimized and discussed. The results show that in the purification step， PCDD/Fs and DL-PCBs are greatly affected by the volume of washing liquid and collection step. PCDD/Fs and DL-PCBs are collected separately. Firstly， the collection solution of PCDD/Fs was measured， and then it was combined with the collection solution of DL-PCBs to complete the determination of DL-PCBs， thereby accurately quantifying DL-PCBs. When the fish was detected， the internal standard recoveries of PCDD/Fs and DL-PCBs were 63.3%-106.8% and RSD was 0.4%-9.6%； The recovery rate of the target substance is 93.6%-114.5%， and the RSD is 0.9%-9.2%， which meets the relevant requirements of the national standard of GB 5009.205. It is suitable for the determination of PCDD/Fs and DL-PCBs in actual fish samples.