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Improving stability is a major aspect for commercial application of perovskite solar cells (PSCs). The all-inorganic CsPbBr3 perovskite material has been proven to have excellent stability. However, the CsPbBr3 film has a small range of light absorption and serious charge recombination at the interface or inside the device, so the power conversion efficiency is still lower than that of the organic–inorganic hybrid one. Here, we successfully fabricate high-quality CsPbBr3 films via additive engineering with NH4SCN. By incorporating NH4 + and pseudo-halide ion SCN– into the precursor solution, a smooth and dense CsPbBr3 film with good crystallinity and low trap state density can be obtained. At the same time, the results of a series of photoluminescence and electrochemical analyses including electrical impedance spectroscopy, space-charge limited current method, Mott–Schottky data, and so on reveal that the NH4SCN additive can greatly reduce the trap state density of the CsPbBr3 film and also effectively inhibit interface recombination and promote charge transport in the CsPbBr3 planar PSC. Finally, the CsPbBr3 planar PSC prepared with a molar ratio of 1.5% NH4SCN achieves a champion efficiency of 8.47%, higher than that of the pure one (7.12%).