The present invention relates to an in-vivo bulking agent which can be used as a medicinal agent for prevention or treatment of at least one disease selected from the group consisting of urinary incontinence, fecal incontinence, and gastroesophageal reflux or as a filler for use in a plastic surgery procedure, and to a preparation method therefor. The in-vivo bulking agent can exhibit a bulking effect when injected into the body and particularly, is highly biocompatible because a first composition in which silicone particles are coated with a zwitterionic polymer having a surfactant property is introduced into a second composition, whereby the bulking agent can inhibit inflammatory reactions in vivo.