The invention discloses a system for preparing a sizing agent. The system comprises a supplying device, a grading device, a first conveying device, a sand sediment trap, a second conveying device, an impurity removing device, a smashing device, a third conveying device and a pulping device, wherein the impurity removing device is arranged in a conveying line of the second conveying device; the third conveying device is connected with the smashing device and the pulping device; the pulping device is connected with sand sediment trap; the sand sediment trap comprises a trap body, an inlet and an outlet; a small-particle-size raw material or slurry formed by the small-particle-size raw material and water enters the trap body through the inlet; the trap body comprises a side wall and a bottom; the outlet is formed on the side wall; and the bottom is provided with at least one bump and/or at least one recess. According to the system for preparing the sizing agent, fine impurities are cleared by using the sand sediment trap; and light impurities are cleared by using the impurity removing device, so that clearing of the light impurities is realized.