The invention relates to the field of semiconductor materials and aims at providing a preparation method of a g-C3N4 nanoparticle/flower-like BiOI composite. The preparation method comprises the following steps: carrying out heat treatment on nitrogen-rich precursor powder, and grinding, so that powdery g-C3N4 is obtained; dispersing into a strong acid solution, condensing, refluxing, and cleaninguntil a product is neutral, so that g-C3N4 nanoparticles are obtained; adding Bi(NO3)3.5H2O and KI into a mixed solution of deionized water and ethylene glycol, stirring and dissolving, then carryingout hydrothermal reaction, and washing the obtained precipitates and then drying, so that flower-like BiOI is obtained; dispersing the flower-like BiOI and the g-C3N4 nanoparticles into the deionizedwater, carrying out ultrasonic oscillation, and then stirring; and centrifuging, then cleaning the precipitates, and drying, so that the final product is obtained. The preparation method provided bythe invention is simple and easy to operate; and the uniformly loaded g-C3n4 nanoparticle/flower-like BiOI composite can be prepared, so that the composite has high photo-generated carrier separationefficiency and high visible light absorption capability, and a new idea is provided for study of the g-C3N4-based nano composite.