The present invention discloses a Y-type discrete polyethylene glycol derivative and a preparation method thereof, which has the advantages of determined molecular weights and the number of chain segments, and can avoid the defect of heterogeneity of a PEG derivative, meanwhile the preparation method has simple steps, mild conditions, without need for strictly anhydrous environment or performing protection and deprotection steps. In addition, the Y-type discrete polyethylene glycol derivative of the present invention may increase the water solubility of the discrete polyethylene glycol, and solve the problem of insufficient water solubility of the discrete polyethylene glycol-modified insoluble drug caused by an increase of the loading capacity.