the uncertainty of four pesticides in rice was evaluated by improved QuEChERS combined with UPLC-MS/MS. According to the relevant provisions and basic procedures of JJF 1135-2005“Evaluation of Measurement Uncertainty in Chemical Analysis”and JJF 1059.1-2012“Evaluation and Expression of Measurement Uncertainty”, a mathematical model for the uncertainty of the content of four pesticides in rice is established and the sources of uncertainty in the determination process are analyzed and synthesized. The results showed that when the contents of four pesticides were 9.18, 8.81, 8.26, 7.95 μg/kg, respectively, the expanded uncertainty was 1.49, 1.65, 1.40, 1.51 μg/kg (k=2, P=95%), respectively. The main factors affecting the determination results were the mass concentration of the measured substance, followed by the repeatability of sample measurement，sample weighing, purification solution dilution, the recovery rate of standard addition, analytical instrument and the volume of extraction solution. This method is applicable to the uncertainty evaluation of the improved QuEChERS combined with UPLC-MS/MS method for the determination of pesticides in rice, so as to provide a scientific basis for the reliability of the detection results of pesticides in rice.