The all-inorganic lead-free perovskite CsBi3I10 has recently emerged as a promising light absorber. However, the poor morphology of CsBi3I10 film remains a critical issue for fabricating high-performance solar cells. In this work, we report an ion substitution strategy by alloying Sb into CsBi3I10, resulting in dramatically improved grain crystallinity and markedly reduced bandgaps for better light utilization. The Sb-substituted CsBi3I10 film composed of highly-crystalline large grains can form a bulk-heterojunction structure with the electron acceptor PCBM to facilitate the exciton separation. With PTAA as the hole transport layer, the solar cell assembled using Cs(Bi0.7Sb0.3)3I10 can afford a champion power conversion efficiency of 1.54% with a high Voc of 0.81 V.