The invention discloses a controlled-release special nitrogen fertilizer for greenhouse muskmelons, and a preparation method thereof. The nitrogen fertilizer consists of S-type controlled-release urea and L-type controlled-release urea in a mass ratio (based on pure nitrogen) of 75-85:15-25, wherein a polymer coating layer of the S-type controlled-release urea comprises low-density polyethylene, linear low-density polyethylene, polystyrene foam, talc and corn starch, and the polymer coating layer of the L-type controlled-release urea comprises waste polystyrene foam, recycled polyethylene, recycled polypropylene and talc. The method for preparing the nitrogen fertilizer comprises the steps of spraying all components in the S-type and the L-type controlled-release urea on the surfaces of urea particles respectively, obtaining two types of controlled-release urea and then mixing the two types of controlled-release urea well. The nitrogen fertilizer provided by the invention is accurate and controllable in the speed and amount of nitrogen release and easier to realize the synchronization of nitrogen release and crop absorption, thereby greatly improving the utilization efficiency of the fertilizer. In addition, the nitrogen fertilizer is relatively low in cost by adopting a large amount of cheap waste plastics, and has high application value in the aspect of fertilizers for greenhouse muskmelons.