The invention belongs to the technical field of biologically active 
polysaccharide preparation, and discloses a preparation method of modified 
pectin. The preparation method comprises the steps that 
pectin is added into deionized water, mixing and 
ultrasonic dispersion are conducted, stirring is conducted to be dissolved to be uniform, and a 
pectin solution is obtained; the pH value of the pectin solution is adjusted with alkali to range from 7 to 12, 
hydrogen peroxide is dropwise added, under 
ultraviolet irradiation, stirring is conducted at 
room temperature, and reacting is conducted; after reacting is completed, the pH of reaction liquid is adjusted with acid to be neutral, the adjusted reaction liquid is added into an 
ethanol-water solution, and stirring, standing 
settling and filtering are conducted; a 
solid product is subjected to washing, 
drying and purifying, and the modified pectin is obtained. Accordingly, by means of an 
ultraviolet photocatalysis-
hydrogen peroxide oxidation modification method, degradation and 
degreasing of the pectin can be achieved through one step, and the preparation method has the advantages of being short in reaction time, free of 
pollution, high in 
product recovery rate and low in 
energy consumption.