The invention relates to a soybean dietary fiber biscuit which is produced by taking soybean dregs as a main raw material and a preparation method thereof. The preparation method comprises: processing the soybean dregs by high-pressure steam; grinding by a 450-500 mesh grinder to obtain soybean dietary fiber; mixing the soybean dietary fiber with auxiliary materials, such as flour, milk, eggs, phospholipid, maltose and the like; and preparing the soybean dietary fiber biscuit by rolling shaping and baking. The soybean dietary fiber accounts for 35-60%, and the soybean dietary fiber biscuit also contains various trace elements and vitamins, such as protein, bifidus factor, isoflavone, phospholipid, unsaturated fatty acid, saponin, calcium, zinc, phosphorus, selenium, iron, magnesium, potassium, VB and the like. Thus, four side effects such as thyroid enlargement, stomach upset and nausea, flatulence, big possibility of uarthritis caused by inherence of soybean dietary fiber and the like are thoroughly overcome and the difficulties of fiber synechia and puffing property as well as rough taste is solved.