The invention relates to the technical field of photocatalytic adsorption, and discloses a Sn3O4-BiOCl heterojunction photocatalytic composite porous adsorption material and a preparation method thereof, wherein the Sn3O4-BiOCl heterojunction photocatalytic composite porous adsorption material comprises the following formula raw materials: Si doped Sn3O4, KCl, Bi(NO3)3, sodium dodecyl sulfate anda modified active carbon material. According to the invention, due to the doping of Si, a large number of oxygen defects formed by Sn3O4 are reduced, so that the band gap of Sn3O4 is narrowed, the Sn3O4 light absorption wave band is expanded, the oxygen defect energy level can capture electrons, and the recombination rate of photo-induced electrons and holes is reduced; the Sn3O4 and the BiOCl form a p-n type heterojunction, so that the separation of photo-induced electrons and holes is promoted; 2-hydroxypropyl-beta-cyclodextrin containing a large amount of hydrophilic groups such as hydroxyland amino and chitosan form a cross-linked product to modify active carbon, the active carbon is complexed with heavy metals such as copper and cadmium and ions thereof to form a chelate, and the hydrophilicity of the composite material is enhanced, so that the cross-linking degree of chitosan is increased through 2-hydroxypropyl-beta-cyclodextrin, and the chemical stability of chitosan is enhanced.