It is shown that the introduction of a mixed cellulose esters with acetrimethyl acetate into films of organometallic perovskite CH3NH3PbBr3 significantly increases their stability while maintaining the optical and photoelectric properties of perovskite. It was found that with the introduction of 10-20 wt% mixed cellulose ester in CH3NH3PbBr3, the resistivity of the composite increases by 5-10 times within 60-70 days, while in pure perovskite films such changes are observed within 10-15 days. When the films are irradiated with a simulated sunlight, the resistance of the samples drops by 2-3 orders of magnitude, and the effect of photosensitivity persist for more than 2 months. The formation of hydrogen bonds between mixed cellulose ester and organometallic perovskite is, in our opinion, the main factor that increases the stability of the composite film in comparison with pure perovskite. The investigated composite films are promising for the design of degradation-resistant perovskite solar cells and light-emitting diodes.