A phase lock loop (PLL) device with correcting function of loop bandwidth and method thereof is related to the method including generating an output signal by a PLL circuit according to a reference signal and a feedback signal, modulating a feedback coefficient to unlock the feedback signal and the reference signal, detecting two valid crossovers of a phase difference between the reference signal and the feedback signal, calculating an oscillation frequency according to the two valid crossovers, and setting a control parameter of the PLL circuit according to the oscillation frequency. The feedback signal is related to the output signal, and there is the feedback coefficient between the feedback signal and the output signal.