With the expansion of oil and gas exploration, protective coatings are being introduced into drilling systems to address the increasingly demanding service environment of ultra-deep well drilling. This paper focuses on the debonding failure of the cemented interface between the drill pipe and the coating. Taking S135 steel grade drill pipe and polyvinylidene fluoride (PVDF) coating as the research object, a finite element model of the coating-drill pipe bonding is established based on cohesive contact theory. Using the surface cohesion behavior and secondary nominal stress damage criterion in Abaqus finite element software, the influence of coating defects on the interface under different coating thicknesses, load types, areas, and shapes is studied. Simulation results show that a coating thickness of 0.6 mm-0.8 mm is optimal; improving interface toughness is better than coating thickness design; under multi-axial coupled stress, the notch has the greatest influence on the interface; the interface life of holes with a radius of 4 mm is the shortest, while large-area defects result in significant coating stress concentration; shallow and wide holes exhibit rapid interface debonding, while deep and narrow holes pose a risk to the coating itself. Based on the simulation results, engineering suggestions are proposed, providing a theoretical basis for the life extension design and safety evaluation of ultra-deep well coating-drill pipe systems.