Abstract: A sensitive ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was developed for the separation and determination diphenylmethane diisocyanate(MDI). Methanol was used for the derived reagent, and diphenylmethane diisocyanate was transformed into diphenylmethane dicarbamate(MDC), then calculated the content of MDI. The sample was effectively separated in 1.5 min on a Waters Acquity BEH C18 column using methanol and water (containing 0.1% formic) as mobile phase under the condition of gradient elution. The detection of compound was performed by MS under single ion monitoring (SIM) mode. The calibration curve is linear over the concentration range of 2—100 μg/L for the compound with detection limits of 1 μg/L， and the correlation coefficient is 0.999 4. The effects of mobile phase systems and the injection volumes of the sample were investigated. The results indicate that the separation efficiency of compounds in acidity mobile phase is higher than other systems, and the large injection has flat head peak in spectrum. Under the optimum conditions, the method was applied in the determination of free monomeric isocyanates in the actual samples.