A water-based viscosity reducer for emulsifying ultra-heavy oil and a preparation method thereof is provided, where the viscosity reducer includes the following components (based on 100 parts by weight): 0.7 to 4 parts of surfactant, 0 to 10 parts of oil phase, and the balance being a water phase; and the preparation method thereof is stirring the surfactant, the oil phase, and the water phase at room temperature for mixing evenly, to form the water-based viscosity reducer for emulsifying ultra-heavy oil. The invention greatly improves the overall surface activity by using the synergistic effect generated by the compounding of surfactants, and can significantly reduce oil-water interfacial tension. It is especially suitable for emulsifying viscosity reduction of alkali-sensitive stratum, has characteristics such as low cost, safety, and simple preparation, and can effectively enhance heavy oil recovery.