The invention discloses a chemically modified electrode for quantitative determination of folic acid and a preparation method of an electrochemical sensor. The method comprises the following steps: S11, fetching multiwalled carbon nanotube, adding concentrated nitric acid and concentrated sulfuric acid and then carrying out ultrasonic treatment so as to obtain a suspension; S 12, adding ethanol into the above suspension, and carrying out centrifugal separation; adding water and carrying out centrifugal separation until pH value of a filtrate is 6-8, and drying sediment to obtain carboxylated multiwalled carbon nanotube; S13, fetching the carboxylated multiwalled carbon nanotube, adding water and carrying out uniform ultrasonic dispersion, and adding glucose to form a solution A; S14, adding silver nitrate into water and stirring and dissolving, dropwise adding concentrated NH3.H2O to form a solution B; S15, adding the solution B into the solution A, stirring, ageing, centrifuging and washing, and drying by a drying oven to obtain an Ag / MWCNTs composite material; and S16, dispersing the Ag / MWCNTs composite material in an organic solvent to obtain a dispersion liquid; and coating and fixing the dispersion liquid on the surface of a cleaned glassy carbon electrode so as to obtain the electrode. The chemically modified electrode prepared by the above method has low detection limit and good selectivity.