The invention relates to a building mortar additive and a production method thereof. The building mortar additive is prepared by the following raw materials in parts by weight: 50-60 parts of sepiolite, 40-50 parts of calcium carbonate, 6-8 parts of lauryl sodium sulfate, 1.2-2 parts of calcium lignosulfonate, 1.6-2.5 parts of triethanolamine, 2-4 parts of dihydroxyethyl dodecylacyl, 2-3 parts ofnonyl phenol polyvinyl ether, 0.2-0.4 part of tributyl phosphate (TBP) and 3-5 parts of ethylene glycol. The production method is as follows: adding the sepiolite and calcium carbonate to a stirrer, and adding the other raw materials to the stirrer for sealed stir for 13-15min to obtain a powdery product. The additive in the invention enables the mortar consistency to be in direction proportion to mortar strength, adapts to use requirements in low temperature environments, and achieves the anti-freezing purpose without adding other anti-freezing materials additionally; and when the additive is used as an additive for whitewash mortar, the mortar can be plastered on the smooth surface of concrete in no need of lime putty and other auxiliary materials, thus solving the difficult problem of cracked and hollow mortar in the process of plastering the concrete smooth surface, having the capability of replacing the lime putty in whitewash mortar completely and realizing low carbon and environment protection.