The invention relates to a preparation method of a zinc smelting slag composite briquette, belonging to the technical fields of metallurgy and chemical industry. The method comprises the following steps: (1) ball milling: respectively carrying out ball milling on lead silver slag, iron alum slag and blast furnace slag to obtain three powders in which the content of particles with the size of less than 200 meshes is greater than or equal to 80%; (2) proportioning: mixing the iron alum slag powder, lead silver slag powder and blast furnace slag powder in a mass ratio of (1.5-3):(0.8-2):1, and mixing Portland cement accounting for 0.5-8 wt% of the total material while adding water until the water content of the mixture is 12-20; and (3) carrying out briquetting on the mixture with a hydroforming machine under the forming pressure of 20-80Mpa to produce a wet briquette, and drying in a drying kiln at 30-180 DEG C to produce the qualified briquette of which the water content is 5-10%. The method can enable the iron alum slag, lead silver slag and blast furnace slag to quickly form, and has the characteristics of simple technique, low reagent consumption, low cost and high formation rate.