The invention relates to a preparation method of a polysulfonamide plate. The technical method comprises the following steps of (1) putting polysulfonamide into a reaction kettle, adding a pre-treatment agent, and soaking fibers; (2) performing ultrasonic treatment on the soaked fibers for once to four times for 10-60 seconds at each time; (3) putting the treated fibers into a drying oven, drying the fibers under the temperature of 100-200 DEG C, taking out the fibers until the weight of the fibers is constant, and putting the fibers into a vacuum drying box for later use; and (4) performing high-temperature die pressing on the treated polysulfonamide to prepare the polysulfonamide plate. By the use of the technical line, the compactness degree of the polysulfonamide plate can be effectively increased, and the interface binding strength between the fibers and resin is enhanced, so that the aim of improving the material performance is achieved.