The invention discloses a bismuth-based composite photocatalyst and a synthesizing method thereof. The synthesizing method includes: mixing bismuth nitrate and cetyl trimethyl ammonium bromide with apolyol solvent, and evenly stirring to form reaction liquid, wherein in the reaction liquid, the mole ratio of the bismuth nitrate to the cetyl trimethyl ammonium bromide is 1:3-1:6; allowing the reaction liquid to have reaction at 90-130 DEG C for 4-24 hours, and performing cooling, solid-liquid separation, washing and drying after the reaction to obtain the bismuth-based composite photocatalyst.The synthesizing method is simple in process, mild in conditions and high in yield, and the synthesized composite photocatalyst is high in photocatalytic activity and good in stability.