The invention provides a green synthesis method of Bi4MoO9. The green synthesis method comprises the steps that (1) Bi(NO3)3.5H2O is mixed with glycerol to obtain a solution A; (2) (NH4)6Mo7O24.4H2O is dissolved in deionized water to obtain a solution B; (3) NH4HCO3 is dissolved in the deionized water to obtain a solution C; (4) the solution B is slowly added into the solution C and stirred to obtain a solution D; and (5) the solution A is added into the solution D in a water bath at 85-95 DEG C, and is stirred and reacts, and then Bi4MoO9 powder is obtained through post-treatment. According to the green synthesis method, NH4HCO3 is adopted as an auxiliary agent, the disadvantages of strong acid, strong alkali and complex process in the existing hydrothermal process of preparing Bi4MoO9 are avoided, the yield is high (more than 90%), the solubility of NH4HCO3 is large, washing is easy, by-products are easy to volatilize, the pollution is small, the source of raw materials is wide, theprice is low, decomposition is easy by heating, and separation is convenient; and Bi4MoO9 prepared by adopting the green synthesis method has excellent visible light catalytic performance and can replace the traditional photocatalyst, such as TiO2, ZnO, and is applied to the treatment of printing and dyeing wastewater, organic wastewater and the like.