To investigate the regulation law and action mechanism of deep cryogenic treatment (DCT) on the interlaminar fracture toughness of carbon fiber reinforced plastic (CFRP), T300 epoxy-based CFRP unidirectional laminates were taken as the research object, and four DCT conditions of 0 h (untreated), 10 h, 50 h and 90 h immersion in liquid nitrogen environment (-196℃) were designed. The evolution laws of Mode I and Mode II interlaminar fracture toughness were systematically studied via double cantilever beam (DCB) test and end-notched flexure (ENF) test, and the influence of treatment duration was analyzed combined with the micro-mechanism. The results show that the DCT duration presents a non-linear regulation characteristic of first increasing and then decreasing on the interlaminar fracture toughness of CFRP, with 50 h being the optimal treatment duration. At this time, the Mode I initial fracture toughness and steady-state fracture toughness reach 0.487 kJ/m2 and 0.713 kJ/m2, increasing by 83.8% and 111.6% compared with the untreated group, respectively; the Mode II steady-state fracture toughness reaches 3.69 kJ/m2, with an increase of 171.4% compared with the untreated group. Moderate DCT (10~50 h) enhances the fiber bridging effect and interfacial bonding strength through the synergistic effect of increasing carbon fiber surface roughness to strengthen mechanical interlocking, optimizing interfacial residual compressive stress to generate clamping effect and inducing moderate cold hardening of epoxy resin to improve modulus. In contrast, excessive DCT (90 h) leads to the embrittlement of epoxy resin and the initiation of interfacial microcracks, which weakens the interlaminar properties of CFRP. The research results provide theoretical support and technical reference for the process optimization and engineering application of CFRP in low-temperature environments.