In order to investigate the fracture performance of aqueous polymer isocyanates (APIs), an analytical expression for the energy release rate of adhesively-bonded three-point bending specimens has been derived from linear elastic fracture mechanics (LEFM) and the energy release rates of nine API samples are evaluated. Experimental results show that a proper loading of polymeric diphenylmethane-4,4'-diisocyanate (p-MDI) as cross-linker can improve the resistance of aqueous polymer latex adhesive layer to cracking. Excess cross-linker cannot maintain such an effect of strengthening and may decrease considerably the energy release rate of API polymer. When the loading of polyisocyanate is less than the critical value, the polymer cross-linked by aqueous emulsified p-MDI has a higher resistance to cracking. In contrast, polymers modified by p-MDI blended with an organic solvent have much better performance.