A circuit calibrating method, applied to an ACS generating circuit, which comprises a plurality of ACS generating units and activates the ACS generating unit corresponding to different DCCs to generate difference ACSs. The circuit calibrating method comprises: (a) determining which one of the ACSs has a large difference from an ideal value thereof; (b) adjusting a number of the ACS generating units, which are activated by a DCC corresponding to the ACS acquired in the step (a), or a next stage of the DCC corresponding to the ACS acquired in the step (a); and (c) generating the ACS to a target circuit, according to the number of the ACS generating circuits adjusted in the step (b).