The present subject matter relates to the drilling fluid field, and discloses a viscosifier for oil-based drilling fluids and a method for preparing the viscosifier for oil-based drilling fluids. The preparation method comprises the following steps: (1) mixing an initiator with an unsaturated amide, an unsaturated organic sulfonic acid, and an unsaturated organic carboxylic acid in an aqueous phase solution and initiating a polymerization reaction with an oil phase solution that contains emulsifier and base oil, to obtain a polymer emulsion; (2) purifying and drying the polymer emulsion to obtain a composition; (3) mix the composition, sepiolite powder, and an acid while stirring, and then drying the obtained mixture. The viscosifier for oil-based drilling fluids disclosed in the present subject matter can make up for the shortage of viscosifiers for oil-based drilling fluids in the art, and has favorable temperature tolerance property, viscosity improvement property and environmental friendliness.