The invention discloses a preparation method of superfine glass fiber, and the method comprises the following steps: S1, soaking glass fibers in a sodium hydroxide aqueous solution, washing, drying, uniformly dispersing in an organic solvent to obtain a solution A, adding diisocyanate and a catalyst, reacting, filtering, and washing a filter cake to obtain an intermediate material; and S2, uniformly dispersing the intermediate material in an organic solvent to obtain a solution B, adding diamine into the solution B in an inert gas atmosphere for reaction, filtering, washing a filter cake, anddrying to obtain the superfine glass fiber. The invention further discloses the superfine glass fiber which is prepared according to the preparation method of the superfine glass fiber. The inventionalso discloses application of the ultrafine glass fiber in a polyimide film. The invention further discloses a low-heat-shrinkage polyimide film. The surface of the prepared superfine glass fiber is grafted with-NH2, and the superfine glass fiber can be combined with polyimide to achieve the effects of reducing thermal shrinkage of the polyimide film and maintaining mechanical properties.