The invention discloses an alkali-activated high-performance water reducer and a preparation method thereof. The alkali-activated high-performance water reducer comprises a composite alkali-activator, a composite water reducer, black liquor of a paper mill, a composite retarder, a composite air entraining agent, a composite thickener, a composite regulator and the balance of water. The preparation method for the alkali-activated high-performance water reducer comprises the following steps of: adding the components and water to a reaction kettle in sequence; and mixing and carrying out a reaction on the components and the water under the room-temperature environment to obtain the finished product. The alkali-activated high-performance water reducer disclosed by the invention can activate the cementitious activity of the mixed materials such as slag, coal ash, gangue and the like, has excellent adaptability to different cements and highly doped concrete, and can greatly reduce the adding amount of cement clinker. The alkali-activated high-performance water reducer has the characteristics of a high water-reducing agent, good peaceability, a remarkable reinforcing effect, a good collapse preventing effect, adjustable setting time and the like. Moreover, the external additive has a simple production process, no pollution, low energy consumption and low cost, can solve an environmental pollution problem of the black liquor of the paper mill, and has good practicability.