The invention relates to a method for regulating a speed of a brushless direct current motor supplied with power by a four-switch three-phase inverter, and belongs to the field of speed regulation of a direct current motor. The method solves the problem that unfixed switch frequency of hysteresis loop modulation in the control of the conventional brushless direct current motor. The method is realized based on a speed loop PI regulator, a speed calculation unit, a current shape function unit, a given current coordinate conversion unit, an alpha-axis current PI regulator, a beta-axis current PI regulator, a flux linkage tracing PWM unit, a feedback current coordinate conversion unit, a four-switch three-phase inverter, a current sensor and a Hall pole sensor, and adopts rotational speed and current double closed-loop control; an outer loop is a rotational speed ring; the outputs of the rotational speed ring are given as current values; the method multiples the current values by the current shape function unit respectively to realize the preset of three-phase 120-degree symmetrical square-wave current, establishes a current closed loop in a two-phase stationary coordinate system by coordinate conversion, and realizes the control of the four-switch three-phase inverter through the flux linkage tracing PWM unit. The method is used for controlling the speed regulation of the brushless direct current motor.