All‐inorganic mixed‐halide perovskite CsPbI3−xBrx materials have attracted extensive attention in optoelectronics due to upgraded stability and tuned bandgap. Unfortunately, undesired halide phase segregation by ion migration under illumination deteriorates photoelectrical performance and long‐term stability of devices. The severity of phase segregation is prone to occur at grain boundaries and relies on surface/interface defects. Herein, a thermal‐pressed (TP) crystal‐growth strategy is proposed to realize sufficient grain growth and boundary fusion simultaneously in a series of CsPbI3−xBrx films. The large‐sized crystals (20–40 µm), self‐fused boundaries, and single‐crystal‐like surface effectively block photoinduced ion migration. The TP‐based CsPbI3−xBrx films maintain excellent long‐life photostability over one year and exhibit markedly suppressed halide phase segregation under continuous laser illumination. Consequently, the TP‐based photodetector devices exhibit superior long‐term stability under laser illumination with different wavelengths.