The invention relates to a preparation method of polyimide fiber. The method mainly solves the problem in the prior art that when a two-step method is used for preparing polyimide fiber, filtering is difficult and spinning is not stable due to polyamide acid solution viscosity which is not stable and the molecular weight which is too high, and viscosity which is too high. The preparation method of polyimide fiber comprises the following steps: polymer raw material diamine, a solvent and a conditioning agent are added into a reactor, dissolving is carried out with the protection of nitrogen, tetracarboxylic dianhydride is added at -10-40 DEG C in order to carry out a reaction for 2-10 hours, and a polyamide acid stock solution is obtained; pressure of the stock solution is reduced, bubbles are removed, and filtering is carried out in order to obtain a polyamide acid spinning stock solution; the stock solution employs a wet method spinning process for carrying out spinning; solidification, washing, oiling, and densification by drying are carried out in order to obtain precursor; imidization and hot-stretching of the precursor are carried out, in order to obtain the polyimide fiber with high compactness and high strength. The technical scheme better solves the problem, and can be applied to industrial production of polyimide fiber.