A time-to-digital converter and a phase difference detection method are disclosed. The time-to-digital converter includes a detection unit and a digital control circuit. The detection unit comprising: a phase detection circuit, a first and a second clock signal are respectively coupled to an identical input terminal of the phase detection circuit and a reference signal is coupled to another input terminal of the phase detection circuit; the phase detection circuit is configured to output a pulse width corresponding to a phase difference between the first or the second clock signal and the reference signal; a filter circuit, coupled to an output terminal of the phase detection circuit; a ring oscillator circuit, coupled to an output terminal of the filter circuit and configured to output an oscillation clock signal corresponding to the pulse width. The digital control circuit is configured to provide the reference signal and receive the oscillation clock signal.