The invention discloses a method for controlling the output current of a switching power supply and a controller for realizing the method, belonging to the switching power supply field. The method comprises the following steps: the output voltage of the switching power supply or an equivalent signal thereof is sampled at the primary side of a transformer, the scaling signal of the output voltage is obtained, meanwhile, the current or an equivalent signal of a primary coil is sampled, the square processing is carried out after the scaling signal of current of the primary coil, and a current square signal to be compared is obtained; the scaling signal of the output voltage is compared with the current square signal to be compared to obtain a current enabling control signal, if the current square signal to be compared is less than or equal to the scaling signal of the output voltage, the current enabling control signal enables the power switch not to be turned off, otherwise, the power switch is turned off through the current enabling control signal. The controller comprises a multiplier or squaring circuit for performing square processing to the current of the primary coil, and a comparator for performing comparison. The invention has the advantages of simple structure and low cost.