The invention provides an additive for polyurethane synthetic leather and a preparation method thereof. The additive is prepared from the following raw materials by weight: 25-28 parts of polyester resin, 7-9 parts of terpene resin, 2-3 parts of white carbon black, 1.2-1.5 parts of soap powder, 1.8-2.2 parts of ethylene-bis-stearamide, 0.8-1.3 parts of a silane coupling agent, 0.6-0.7 part of betaine, 1.2 parts of a dispersant, 0.6 part of a fluorine drying agent, and 60-65 parts of water. The additive provided by the invention can be added into synthetic leather to use, is harmless to the environment, also has no harm to human health, and can improve the water resistance and stability of synthetic leather. The finally obtained synthetic leather has the characteristics of antistatic properties, low irritation, sterilization, low toxicity, emulsification dispersion, chemical stability and biodegradability, etc.