A clock generating apparatus includes: a time-to-digital converter (TDC) arranged to convert a timing difference between a reference clock and a variable clock to generate a digital value; a calibrating device arranged to generate a control signal according to the digital value and the reference clock; a controllable oscillator arranged to generate an oscillating signal according to the control signal and the digital value; and a feedback device arranged to generate the variable clock to the TDC according to the oscillating signal, and the calibrating device calibrates the controllable oscillator to make the oscillating signal have a target oscillating frequency.