The work aims to effectively solve the problem of low viscosity of resin adhesives and reduce the release of formaldehyde in plywood that pollutes the environment and to make the prepared resin adhesives better meet the requirements of indoor plywood products. The melamine-glyoxal-urea resin adhesive (MGU for short) was prepared with melamine (M), glyoxal (G) and urea (U), the MGU resin adhesive (PVA/MGU for short) was optimized through modification of orthogonal experiments with polyvinyl alcohol (PVA) and the gluing experiments were carried out. The best preparation process of MGU resin adhesive included the molar ratio of raw materials of G : M : U=1 : 0.18 : 0.10, the reaction temperature of 65 ℃, the reaction time of 70 min, and the PVA-modified MGU resin adhesive with a mass fraction of 10%. The pressed plywood had the best bonding strength of 0.73 MPa, which met the requirements of GB/T 17657—2013 Class II plywood. The viscosity of the PVA/MGU resin adhesive was 36.2 s, which was 27% higher than that of the MGU resin adhesive, and the content of free formaldehyde was 0.011%, which largely met the requirements of plywood use. In addition, due to the low content of free formaldehyde, the MGU resin adhesive had broad application prospects. According to the FT-TR analysis, PVA is chemically bonded to the MGU resin adhesive. From the TG test results, the thermal stability and water resistance of the PVA/MGU resin adhesive are significantly improved. According to the DSC analysis, the curing temperature of the PVA/MGU resin adhesive is reduced compared with that of the unmodified one.