Polytetramethylene glycol (PTMG)-isophorone diisocyanate (IPDI) and polycarbonate (PC)-IPDI waterborne polyurethane (WPU) films were prepared and crosslinked with 2-methyl-1-aziridinepropionate (aziridine compound). The crosslinked and uncrosslinked samples were hydrolyzed with water vapor at 120 °C in an autoclave. The effects of the introduction of aziridine crosslinking on the hydrolysis and mechanical properties of the samples were investigated. When the content of aziridine crosslinking agent increased, the fracture stress of each sample decreased before hydrolysis, but the retention rate of mechanical properties after hydrolysis was significantly increased than that of uncrosslinked sample. When the PTMG-WPU film was crosslinked at an aziridine content of about 1 pph and hydrolyzed at 120 °C within 24 h, the retention rate of fracture stress was greatly higher than that of the uncrosslinked sample. However, PC-WPU samples did not show a significant crosslinking introduction effect. Both the chemical structure of WPU polymer and physical structure formed during film preparation affected the mechanical properties before and after hydrolysis and the effect of crosslinking introduction.