The invention relates to an alkali slag concrete composite activator and a preparation method of a concrete mixture. The composite activator in alkali slag concrete is made by mixing solid sodium hydroxide with a composition of one or two of solid alkali metal carbonate and alkali metal bicarbonate; the preparation method of the mixture includes the following steps: first, preparing activated powdery material, the composite activator and mixing water material; second, mixing materials of solid components; third, after adding all the solid components, adding mixing water and stirring; by using the composite activator and the preparation method thereof according to the technical scheme, it is possible to overcome the technical defect that sodium hydroxide in alkali slag concrete is currently dissolved sodium hydroxide so that the production process has a complex flow; the solid activator need not be dissolved and cooled first, the production process is simplified, production cost is reduced, the applicable range is wide, and there is no need for additive retarders.