The long-term emission of formaldehyde from wood-based composites bonded with urea- formaldehyde(UF) resin is the major pollution source in home environment. As such, producers at present have lowered the formaldehyde-to- urea mole ratio to reduce the formaldehyde emission, However, this approach resulted in significant reduction in the branched structure and crosslinking degree of UF resin, ultimately deteriorate bonding performance and water resistance of UF resin. The decisive factor for the comprehensive performance of UF resin lies in microstructure formation. Therefore, optimizing and modifying the molecular structure is always effective to balance the curing rate, formaldehyde emission, and bonding performance of UF resin. In this paper, the mechanisms involved in optimizing and modifying UF resin structure using modifiers such as melamine, glyoxal, and dendrimers were summarized, the influence of modification methods and processes on the curing performance, bonding strength, and formaldehyde emission of UF resin was also discussed. Furthermore, the main challenges for the future research and development of modified UF resin were proposed.