Thermal deformation and local burnout are common machining defects of PEEK. At present, PEEK is mostly machined by conventional flood cooling, and there are relatively few studies based on LN2 cooling. In this paper, we conducted cryogenic turning experiments on PEEK and investigated the effect of LN2 internal injection cooling on surface quality and found for the first time that the fracture form of PEEK changes from microporosity-dominated plastic fracture to brittle fracture under the strong cooling effect of LN2. This study analyzes the influence of cutting parameters on machining quality through orthogonal experiments, and selects optimal parameters accordingly. Comparative turning experiments are conducted under three different conditions - dry, conventional flood cooling, and LN2 internal injection cooling - to obtain machining information such as cutting temperature, surface roughness, and machining accuracy under each condition. Based on microscopic morphological analysis, we propose a surface formation mechanism for cryogenic turning of PEEK. The study demonstrated that the use of LN2 internal injection cooling substantially decreases the cutting heat, suppresses the thermal deformation of the material, and promotes brittle fracture in PEEK, thereby enhancing machining quality and accuracy. The surface roughness of the workpiece decreased by 53.5 % and 39.2 %, respectively, while the machining accuracy error decreased by 40.4 % and 37.8 % compared to dry cutting and conventional flood cooling methods. These findings provide valuable guidance for efficient machining of PEEK.