The invention belongs to the field of composite material fiber shaping agent materials and relates to a low-viscosity high-thermal resistant polyimide fiber shaping agent and a preparation method thereof. The shaping agent is of a structure with amido acid, monoester carboxylic acid ammonium salt and imide, 4-phenylacetylene phthalic anhydride is adopted as an end sealing agent, nonlinear mixed diamine and 2,3,3',4'-biphenyl dianhydride are adopted as monomers, and the molecular weight is optimized. The shaping agent is prepared through two steps of shaping agent solution synthesis and thermaltreatment, the preparation method is simple and high in preparation efficiency, has a good fiber shaping effect, fiber viscosity, manufacturability and thermal resistance and can be applied to shaping processes such as high-temperature resistant polyimide resin-based composite material resin transfer molding (RTM).