A method is provided for controlling an external control type fan clutch. The method controls the On/Off of an electromagnet-activated valve for an oil feed adjusting hole. The method uses a PID control on the basis of radiator cooling liquid temperature, fan speed, transmission oil temperature, vehicle speed, engine speed, compressor pressure of an air conditioner, and an On/Off signal of the air conditioner. Individual gains of the PID control are determined by a gain matrix containing the optimum fan speed demanded by the engine, the actual metered fan speed, and the engine speed. The PID control is made on the basis of the gains to that a control signal is outputted. The control method can improve mileage, raise the cooling efficiency of an air conditioner and suppress fan noise.