Polyetheretherketone (PEEK) gears are high-performance polymer gears that are widely used in automobiles, drones, robots, and other fields. However, the complex failure mechanism of PEEK gear and the failure forms change with load lead to a shortage of a reasonable design basis in power transmission applications. Aiming at the unclear failure mechanism and transition of failure forms of PEEK gear, carried out the fatigue performance test of PEEK helical gear pair under oil injection lubrication, and calculated the contact stress and bending stress of PEEK gear, together with analyzing the failure by scanning electron microscopy and other equipment. Experimental results that the failure forms of PEEK gear under oil injection lubrication are mainly tooth surface pitting failure and tooth root fatigue fracture, discussing the failure mechanism of PEEK gear tooth surface pitting and tooth root fatigue fracture under oil injection lubrication. Simultaneously, discovering the critical transition relationship between PEEK gear contact fatigue failure and bending fatigue failure, and proposed an evaluation method for the failure form of PEEK gear. When the ratio of contact stress to bending stress of PEEK gear is lower than 1.02, PEEK gear mainly suffers from root fatigue fracture; when the ratio of contact stress to bending stress is higher than 1.1, PEEK gear mainly suffers tooth surface pitting failure; and when the ratio of PEEK gear contact stress to bending stress is between 1.02-1.1, there is a critical failure transition point between tooth surface contact fatigue and root bending fatigue.