The invention provides a preparation method for synthesizing a SnO2/2D g-C3N4 composite photocatalyst through an electrostatic self-assembly process, belonging to the field of photocatalysis. The preparation method comprises the following steps: carrying out thermal oxidation stripping on bulk phase g-C3N4 in an air atmosphere at a calcination temperature of 450-550 DEG C for 4-12 h to obtain 2D g-C3N4; and ultrasonically dispersing 2D g-C3N4 into an ethanol solution, adding nanoscale SnO2, carrying out stirring for 10-14 h, and performing electrostatic self-assembling to obtain the SnO2/2D g-C3N4 composite photocatalyst. In the method, spherical SnO2 is spontaneously assembled on the surface of 2D g-C3N4 through electrostatic attraction to form a heterojunction structure, and the preparedcomposite photocatalyst has the characteristics of high charge separation and high photocatalytic activity, and can quickly degrade organic pollutants under visible light.