The use of a pseudo-halide anion, such as thiocyanate (SCN–), as an additive in the composition-engineered perovskite film is verified and its impact on the perovskite solar cell (PSCs) performance is investigated. The perovskite precursor added with a small amount of formamidinium thiocyanate is deposited by a one-step solution process to prepare the perovskite film. We observe a significant enlargement in domain size after the incorporation of thiocyanate ions in the perovskite film. Moreover, a trace amount of thiocyanate groups across the perovskite film measured by the time-of-flight secondary ion mass spectrometer identifies the thiocyanate existing inside the perovskite bulk, especially near the film bottom. Replacement of halide with thiocyanate groups in the perovskite framework effectively suppresses bulk recombination in the perovskite film, leading to an improvement on the open-circuit voltage (V OC) and fill factor for the pseudo-halide-based PSCs. Our studies confirm the existence of SCN– in the final film and the passivation effect of SCN– for enhancing the device performance.