Formaldehyde in Chinese cabbage was derivated with 2,4-dinitrophenylhydrazine, extracted by acetonitrile and tested by liquid chromatography tandem mass spectrometry. Results showed that the detection limit of formaldehyde in this method was 3.06μg/Kg. When the spiking amout in Chinese cabbage was 1-18mg / Kg, the formaldehyde amout was linearly correlated to peak area. (r2= 0.994). When the adding amount of Formaldehyde was 0.5,1,5 mg/Kg, recoveries were between 102% to 105%, and the relative standard deviations were between 3% and 8% with good stability and precision.