Abstract: The multi-co-condensed resins of melamine−phenol−urea−glyoxal(MPUG) with different molar ratios of raw materials of melamine(M), phenol(P), urea(U) and glyoxal(G) have been successfully prepared under weak acid conditions. The basic property, wettability and bonding property of the MPUG resins were fully examined, and then the synthetic process of the co-condensed resins was optimized. Fourier transform infrared spectroscopy(FTIR), ¹³C nuclear magnetic resonance spectroscopy(¹³CNMR) and X-ray photoelectron spectroscopy(XPS) were applied to characterize the resins' structure. The results showed that the addition of M and P improved obviously the bonding performance of UG resin, especially for the water resistance of the plywood. The three-layer poplar plywood bonded with MPUG resin with the molar ratio of raw materials(M∶P∶U∶G) of 0.2∶0.1∶1.0∶1.4 has optimum bonding strength. The dry bonding strength, wet bonding strength in cold water for 24 h and wet bonding strength in 63 ℃ hot water for 3 h are 1.62 MPa, 0.75 MPa and 0.43 MPa, respectively. Moreover, the resin and its plywood are of free-formaldehyde emission and can be directly used for indoor under dry condition.