The invention relates to single-component water-based black paint for an automobile and a preparation method thereof. The single-component water-based black paint for the automobile is prepared by mixing three components A, B and C according to the weight ratio of 4 to (4 to 7) to (0.05 to 0.2), wherein the component A is prepared from the following components in parts by weight: 3 to 7.5 parts ofhigh-pigment carbon black, 15 to 35 parts of water, 3 to 10 parts of methyl ethylene glycol, 0.3 to 2.5 parts of base material wetting agent and 1.5 to 7.5 percent of dispersant; the component B is an aliphatic polyurethane dispersion body; the component C is a rheological agent. According to the single-component water-based black paint for the automobile, the compatibility problem of raw materials is remarkably improved, and the flowability, storage stability, luster, blackness and a blue phase are improved.