Abstract: Reducing volatile organic compounds (VOC) contents in waterborne wood coatings is challenging. Modified aqueous polyurethane dispersion (b-PUD) was prepared by blocking polyurethane prepolymer with monohydric alcohol. The effects of blocking agent category, NCO blocked percentage, NCO/OH molar ratio and soft segment polyol type on assistant-film-forming performance of b-PUD were discussed. The results show that b-PUD modified by dipropylene glycol butyl ether (DPnB) has the best assistant-film-forming ability which increases with the increase of blocked percentage. In addition, the assistant-film-forming ability of b-PUD with polyether glycol as the soft segment is better than that of polyester glycol, and polyether N210 shows the best performance. When NCO blocked percentage is 80%, molar ratio of NCO/OH is 1.5 and b-PUD adding percentage is 25%, the minimum film forming temperature (MFT) of the composite emulsion is 2.9℃ and the VOC content of the waterborne transparent primer based on b-PUD is 28 g·L^-1. This coating has excellent sanding property and high transparency.