The invention discloses an automatic frequency tuning circuit of an active RC filter. The automatic frequency tuning circuit comprises comparators, a constant current source, a reference voltage generating circuit, a switched capacitor circuit and a digital logic control circuit. The automatic frequency tuning circuit is characterized in that the reference voltage generating circuit generates first reference voltage, second reference voltage and third reference voltage which are input to the first comparator, an operational amplifier and the second comparator respectively, voltage generated by the constant current source on the switched capacitor circuit is input to the operational amplifier, and the digital logic control circuit receives signals output by the first comparator and the second comparator, conducts operation treatment, outputs control codes to the switched capacitor circuit and adjusts the equivalent capacitance of the switched capacitor circuit. The automatic frequency tuning circuit is simple in implementation method, low in cost, small in volume, low in power consumption, high in tuning precision, good in application prospect and capable of being widely used in fields such as electronics and communication.