The invention relates to a urea-formaldehyde resin and a preparation method thereof. The preparation method comprises the following steps: (1) preparing an aqueous formaldehyde solution; (2) preparingan alkali adjustment solution; (3) adding the first batch of urea, polyvinyl alcohol and an antifoaming agent into the aqueous formaldehyde solution, carrying out uniform mixing, adding alkali to adjust a pH value to 8 to 9 and carrying out a reaction for 30 to 40 min; (4) adjusting a pH value to 4 to 5, carrying out a reaction for 30 to 40 min, adding the second batch of urea and melamine when asample becomes cloudy after dropped into water, and when another sample becomes cloudy after dropped into water, adding alkali to adjust a pH value to 5-6, adding the third batch of urea, and carrying out a reaction at 80-90 DEG C for 30-40 min; and (5) carrying out cooling to 70-75 DEG C, adding the fourth batch of urea, adding alkali to adjust a pH value to 8 to 9, and then carrying out coolingand discharging to obtain the urea-formaldehyde resin. The preparation method of the invention adopts a weak alkali-weak acid-weak alkali process and utilizes a cellulosic ethanol fermentation wastewater and waste residues for preparation of the urea formaldehyde resin; and the preparation method is simple in process, and the content of free formaldehyde is low.