The present disclosure discloses an ultrasonic-assisted method of extracting a pectin rich in RG-I. The method includes: (1) dispersing citrus peel powders in an alkaline solution containing sodium borohydride; (2) placing a solution obtained in step (1) in a water bath of 25 to 40° C., and performing an ultrasonic treatment on the solution obtained in step (1); (3) centrifuging the solution subjected to step (2) at a speed of 6000-8000 rpm to remove residues, retaining a supernatant, and precipitating a pectic polysaccharide with absolute ethanol; and (4) adding water to the pectic polysaccharide precipitant subjected to step (3) to obtain a redissolved solution, transferring the solution to a dialysis bag with a molecular weight cut-off of 3000 to 3500 Da, dialyzing for 48 hours, and freeze-drying in a vacuum freezer to obtain the pectin rich in RG-I.