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Abstract Energy-band gradient halide perovskites are highly desired candidates for fabricating high performance optoelectronic devices. Here, we demonstrate that a mixed halide perovskite single crystal undergoes phase segregation in the longitudinal direction under above-bandgap light illumination. As a result, a micron thick layer with vertically gradient halide composition and thus graded valence band edge is generated at the crystal surface. The resultant gradient structure can facilitate the hole extraction at its interface with a hole transport layer. The longitudinal phase segregation of mixed halide perovskite single crystal is likely driven by abundant defects at the surface. Moreover, the segregation rate is increased in air compared to nitrogen probably due to the combined effect of oxygen and moisture. These findings not only deepen the understanding of phase segregation mechanism in mixed halide perovskite, but also indicate a promising avenue of fabricating vertically energy-band gradient perovskite and enhancing the perovskite-based optoelectronic device performance.