A square-wave three-phase brushless permanent magnet direct current motor provided with a large-teeth-and-small-teeth structure and the assembling method thereof. The number of the magnetic poles on a rotor core is 2P. The number of the slots in a stator core is Z. Z teeth on the stator core are composed of Z/2 large teeth (6) and Z/2 small teeth. An insertion slot (11) is provided on each yoke between two adjacent large teeth (6). Each small tooth is formed by inserting an end part of each small tooth core (8) into one of the insertion slots (11). Each large tooth (6) occupies an electrical circumferential angle falling within a range of 150 DEG to 234 DEG. Each small tooth together with the notches on its two sides occupies an electrical circumferential angle falling within a range of 90 DEG to 6 DEG. The sum of the electrical angles of one large tooth (6) and one small tooth is 240 DEG. Three-phase windings are wound on Z/2 large teeth (6) separately, and Z/6 concentrated windings are arranged at each phase. During assembling, firstly, the windings are wound on the large teeth (6) separately, and then the small teeth are embedded. It is convenient to wind the windings in the motor. The motor can be used as a servo motor or a torque motor.