The invention discloses a preparation method of a Z-type g-C3N4@Ag@Ag3PO4 composite photocatalyst. The preparation method comprises the following steps of firstly, putting urea into a ceramic crucible with a cover, putting into a muffle furnace, and calcining under air atmosphere, so as to obtain a g-C3N4 semiconductor material; by using the g-C3N4 as a carrier, respectively using silver nitrate and sodium dihydrogen phosphate as a silver ion source and a phosphate ion source, respectively using glucose and sodium borohydride as reduction agents, and using iron nitrate nonahydrate as an oxidant, finely preparing the Z-type g-C3N4@Ag@Ag3PO4 composite material with Ag nanoparticles between the g-C3N4 and Ag3PO4. The preparation method of the Z-type g-C3N4@Ag@Ag3PO4 composite photocatalyst has the advantages that the method is simple, the control is easy, the operation is convenient, the cost is low, the green and non-pollution effects of raw materials are realized, and the like; the spectrum absorbing range is effectively widened, the compounding of photon-generated carriers is effectively decreased, and the application prospect and potential practical value are good.