The invention relates to a paper shredder and a working method thereof. The paper shredder comprises a shell, a paper feeding assembly, a paper treating assembly, a discharging assembly, a power assembly and a controller. The paper feeding assembly comprises a storage box, a paper feeding table, a paper conveying roller and a paper feeding roller, and a guide plate is arranged behind the paper feeding roller; the paper treating assembly comprises a traction roller and a steel burr roller, dust collecting pipes are arranged on the two sides of the burr roller, and a painting spraying nozzle, a drying assembly, a paper shredding sensor and a paper shredding cutter roller are sequentially arranged below the steel burr roller; a dust collector, an upper material liquid box assembly and a lower material liquid box assembly are arranged below the controller; the discharging assembly comprises a paper shredding cavity with an ultrasonic generator, a discharging screw and a discharging motor, and a discharging sensor is arranged in the paper shredding cavity; the power assembly comprises a paper shredding motor, and all roller shafts are connected with the paper shredding motor through paper shredding synchronous belt; and the controller is connected with all elements through wires. The invention further discloses a working method of the paper shredder. By means of the paper shredder and the working method, before paper shredding, earlier stage leakage-proof treatment can be carried out, shredded paper reaches the micron order after shredding is carried out, and information leakage is prevented.