The invention provides a method for preparing ultrafine polysulfonainide fiber. The method comprises the following steps: dissolving the polyphenylene sulfone terephthaloyl amide master batch prepared by a conventional method in a strong polar solvent to form 11 to 25 percent spinning solution; stirring the spinning solution for 30 to 90 minutes at a high speed and filtering the spinning solution; heating the spinning solution to a temperature of between 40 and 150 DEG C till regular vacuole occurs in the spinning solution; performing rapid pressurization on the spinning solution in a solution storage pool through an air duct; acting on the heated spinning solution by utilizing a metal electrode; after electric field force exceeds the surface tension of the spinning solution, forming massinjection streams on the top end of the vacuole; and subsequently, rapidly stretching and differentiating the injection streams to obtain the ultrafine polysulfonainide fiber. When the method is usedto prepare polysulfonainide yarn, the production speed can reach between 50 and 200m/min, and the diameter of the polysulfonainide fiber is between 0.6 and 2.2 mu m and is only 1/8 to 1/10 of that oftraditional polysulfonainide.