The invention relates to the field of casting auxiliary materials, and particularly relates to a hardening accelerating agent for 3D (Three-Dimensional) molding sand printing thermohardening phenolic resin and a preparation method thereof, wherein the accelerating agent is prepared from the following components in percentages by weight: para-toluenesulfonic acid, tartaric acid, formic acid, malic acid, acrylic acid, methanol, ethanol and aviation kerosene. The preparation method comprises the following steps of pumping the para-toluenesulfonic acid, the tartaric acid, the malic acid, the methanol and the ethanol in an enamel reaction kettle by using a vacuum pump, starting agitation, and agitating an obtained first mixture for 60min; adding the formic acid and the acrylic acid into the agitated first mixture, and continuously agitating an obtained second mixture for 30min; adding the aviation kerosene into the agitated second mixture, continuously agitating an obtained third mixture for 30min, obtaining the hardening accelerating agent for the 3D molding sand printing thermohardening phenolic resin, and inspecting and packaging the hardening accelerating agent for the 3D molding sand printing thermohardening phenolic resin. The hardening accelerating agent has the advantages of being low in viscosity, few in free acids, low in moisture and low in acidity, being capable of realizing low-temperature rapid hardening and stable performance, and the like.