As organic solar cells (OSCs) work under illumination, the light-induced degradation issue is an inevitable barrier to the commercialization of OSCs. Herein, the photo-degradation mechanism of high-efficiency OSCs based on PM6:Y6 or its derivatives is investigated under different wavelength illumination in a nitrogen atmosphere. The PM6:Y6 aged device after 1 hour burn-in degradation exhibits significant losses in the open-circuit voltage (V_OC), fill factor (FF) and power conversion efficiency (PCE) with the short-circuit current density (J_SC) nearly unaffected. The V_OC loss and FF loss are mainly ascribed to the reduced charge carrier mobility, especially the hole mobility, the weakened current extraction capability, and the increased non-radiative recombination in the aged device. The aged devices without a band-pass filter and with an ultraviolet filter show more severe photo-degradation than those with other filters, which shows that the ultraviolet portion contributes the most to the device photo-degradation. Based on the comparison of the donor-exposed devices, the acceptor-exposed ones, and the blend-exposed ones, it is inferred that the photo-degradation of PM6 dominates the photo-degradation process of the PM6:Y6 based devices. This work offers a new avenue for exploring the shortcoming that limits the device photo-stability, and unveils that one facile solution to improving the device photo-stability is enhancing the photo-stability of the donor.