The long-term degradation of epoxy as the matrix and adhesive serviced in harsh environments plays a key role in engineering applications. Understanding how to improve the toughness and durability of epoxy through reasonable material replacement and design is significant to prolong the service life of engineering structures. In the present paper, thermoplastic polypropylene and thermosetting epoxy were exposed in a coupling environment of elevated temperature, water immersion and sustained bending loading. The evolutions of mechanical and thermal properties were further analyzed and compared. Long-term life prediction was conducted to evaluate the corrosive resistances of polypropylene and epoxy. It can be found that polypropylene has better hydrophobic behavior compared to epoxy. At 80 °C, the ratios of the diffusion coefficient and saturated water uptake between the two matrices were 114.4 and 2.94. At the longest immersion time of 90 days, the degradation percentages of tensile strength were 4.7% (40 °C), 7.5% (60 °C) and 8.8% (80 °C), respectively, which had the higher strength retention (>90%). The maximum strength increase in the multiples of polypropylene/epoxy and polypropylene/polyurethane was 1.95 and 1.75, respectively. The bending loading led to a maximum increase in tensile strength (~1.47%) owing to the oxygen isolation effect. The degradation mechanism was attributed to the active functional groups from the production process reacting with oxygen, resulting in the fracture of the local chain segment. By comparison, water molecules reacted with the hydroxyl groups or interrupted the intermolecular Van der Waals force/hydrogen bond of the epoxy, resulting in irreversible hydrolysis and property degradation. Through the comparison, it can be found that polypropylene and its composites have outstanding properties compared to epoxy, which can make them achieve great application prospects in engineering applications when considering a complex service environment.