An apparatus and a method for achieving lock-in of a phase-locked loop (PLL) are disclosed. The PLL receives a reference clock and generates an output clock according to the reference clock. The method comprises: adjusting an oscillation frequency of a controlled oscillator of the PLL close to a desired frequency by counting the number of rising edges of a first clock in a number of a second clock cycles; aligning a rising edge of a third clock and a rising edge of a fourth clock by temporarily changing the oscillation frequency of the digitally controlled oscillator; and locking the phases of the third and fourth clocks by a phase detector of the PLL, wherein the first and the third clocks correspond to the output clock and the second and fourth clocks correspond to the reference clock.