The invention relates to a preparation method and application of a Bi/Bi2SiO5 photocatalyst, belongs to the fields of controllable preparation of a Bi/Bi-based photocatalytic material composite system, CO2 reduction resource conversion and utilization and artificial photosynthesis, and solves the problems of narrow light response range of Bi2SiO5, high photoelectron-hole combination rate and the like. In the preparation method, Bi(NO3)3.5H2O and Na2SiO3.9H2O are respectively taken as a bismuth source and a silicon source, ethylene glycol and deionized water are taken as solvents, a NaOH solution is taken as a pH value regulator, a one-step hydrothermal method is utilized, the pH value of a reaction solution is adjusted to be 9-14 by regulating the concentration of the NaOH solution, and the high-purity Bi/Bi2SiO5 photocatalyst can be obtained. Finally, activity, selectivity and stability of Bi2SiO5 for photocatalytic reduction of CO2 are improved, and a new method and a new applicationare provided for preparation of a high-efficiency novel elemental Bi/Bi-based photocatalytic material composite system.