A frequency generating system including a phase-locked loop (PLL) and a control signal generation unit is provided. The PLL outputs a phase-locked clock and controls a voltage-controlled oscillator (VCO) therein by using a dual-path architecture. The VCO includes a varactor. The control signal generation unit is coupled to the PLL and disposed in one of the dual paths. The control signal generation unit provides an up voltage, a down voltage, or a middle voltage as a control signal to control the VCO according to an up signal and a down signal of the PLL. The control signal generation unit provides the middle voltage in response to an electrical characteristic of the varactor to compensate the control signal.