The work aims to analyze the critical control points in determination of plasticizer migration from food contact materials though evaluating the uncertainty of measurement. Under the condition of 50% (volume fraction) ethanol of simulated solution, 60 ℃ of the bath temperature and 1 h immersion time, the gas chromatography-mass spectrometry (GC-MS) was employed to determine the migration of the plasticizer dioctyl phthalate (DEHP) in the plastic cup, which belongs to contact materials. Moreover, the uncertainty factors in the detection process were evaluated and quantified. The major factors contributing to the measurement uncertainty in the detection process were the sample uniformity, the repeatability of the detection instrument and the fluctuation of bath temperature during migration. The relative standard uncertainty of the three factors were 9.55×10−2, 5.67×10−2 and 4.58×10−2, respectively. The uniformity of the sample and repeatability of the testing instrument should be carried out in three parallel tests, and temperature fluctuation allowance of the electric thermostatic water bath pot used for detection should be ±1.0 °C. Each critical control points, should be controlled strictly to guarantee the accurate and precise determination result of plasticizer.