The PM-based adhesives were prepared by crosslinking modification with diphenylmethane diisocyanate (MDI) as a crosslinking agent. The viscosity and water resistance of the modified PM-based adhesive were improved significantly, making it beneficial to the application of label industry. This paper investigated the properties of PM-based adhesives, including viscosity, solid content and water resistance time. Viscosity and solid content of the modified adhesives improved observably as MDI was introduced, and its water resistance time was up to 77 h when the modifier was added at 15% (based on PM), which met the industry requirements of Beer labels in China. In addition, FTIR and XRD were used to characterize the physical and chemical interactions in the cured adhesives. The thermal stability of the cured adhesives was determined by TGA, and SEM was used to observe the microstructure of the cured adhesives. The characterization and analysis results of the cured adhesives showed that incorporating MDI into the adhesives formed dense crosslinking structure because of chemical reactions between MDI and active groups on the PM protein molecules and introduced hydrophobic group into the adhesives, preventing water intrusion effectively, thus improving the water resistance of the cured adhesives.