Abstract: A joint detection method with gas chromatograph-mass spectrometer (GC-MS) for uncertainty evaluation of Fipronil residue in cabbage was introduced, and a mathematical model for evaluating the uncertainty was established, and then the uncertainty origins was determined, the uncertainty components were quantized and synthesized. The results showed that the uncertainty origins in determination were composed of standard fitting curve, replications, and recovery rate. When Fipronil content in cabbage was 0.0821 mg/kg, the extend uncertainty was U=0.00492 mg/kg (k=2).