Organometal halide perovskites exhibit a bright future for applications in solar cells, as efficiency has achieved over 22%. The long-term stability remains a major obstacle for commercialization. Here, it is found that three cationic compositional engineered perovskites, MAPb(I0.83Br0.17)(3), FA(0.83)MA(0.17)Pb(I0.83Br0.17)(3), and Cs-0.1(FA(0.83)MA(0.17))(0.9)Pb(I0.83Br0.17)(3), undergo severe degradation under white-light illumination in ultrahigh vacuum (UHV) environment, but the rate of degradation is significantly lower for the mixed cation perovskites. This is attributed to the defect-induced trap states that trigger the strong coupling between the photoexcited carriers and the crystal lattice. The observed behavior supports the view of the mixed cations suppressing the photoinduced degradation. It is further demonstrated that UHV environment remarkably accelerates the degradation of the perovskite films under illumination, which delivers a very important message that the current hybrid perovskite materials and their optoelectronic devices are not suitable for application in outer space. Moreover, the applied UHV environment can be an accelerated test method to estimate the photostability of the perovskites.