Abstract: 【Objective】Phosphomolybdic acid composite membrane modified electrode was prepared, and its electrochemical properties and the determination of vitamin C content in fruits and vegetables were explored, so as to provide a theoretical basis for the determination of vitamin C in dark fruits and vegetables.【Method】The phosphomolybdic acid composite film modified electrode was prepared by electrochemical deposition at E=+0.26 V constant potential, and its electrochemical properties were tested under different conditions. Finally, fruits and vegetables were used as the research object to measure the effect of phosphomolybdic acid composite film modified electrode on the determination of vitamin C content.【Result】The prepared phosphomolybdic acid had a stable Keggin structure, and the phosphomolybdic acid modified electrode had a reversible redox peak within -0.4 to +0.6 V. The electrocatalytic ability of the phosphomolybdic acid composite film modified electrode was positively correlated with the phosphomolybdic acid concentration, and the electrochemical catalytic activity was better in a high acid environment. The modified electrode had obvious catalytic activity for the oxidation of vitamin C, and the catalytic activity of oxidation increased with the increase of the oxidation degree and sweep rate of phosphomolybdic acid. During the continuous increase of vitamin C, the modified electrode could quickly respond to vitamin C, and the shortest response time was only 3 s. Using this electrode as a working electrode, the vitamin C contents of three samples of orange, carrot and green pepper were 45.32, 71.16 and 96.99 mg/100 g, the relative standard deviation(RSD) of the average of the three determination results did not exceed 5.0%, and the minimum detection limit was 1.1×10^-8 mol/L(S/N=3).【Conclusion】The phosphomolybdic acid composite film modified electrode has good oxidation catalytic activity, and has the characteristics of high sensitivity, low detection limit and small error in measurement results for the detection of vitamin C in dark fruits and vegetables.