Abstract: The present disclosure provides a multi-phase clock signal phase difference detection and calculation circuit and method, and a digital phase modulation system. The detection and calculation circuit includes an auxiliary digital-to-time conversion circuit, a main digital-to-time conversion circuit, a phase detector, and a state machine. The auxiliary digital-to-time conversion circuit selects a first phase clock signal and outputs an auxiliary clock signal, adjusts the phase of the auxiliary clock signal; the phase detector detects the phases of the auxiliary clock signal and a target clock signal output by the main digital-to-time conversion circuit; the state machine adjusts the phase of the auxiliary clock signal, and adjusts the phase of the target clock signal. When the phase difference between the two signals is zero, the amount of phase adjustment by the main digital-to-time conversion circuit is the phase difference between the first phase clock signal and the second phase clock signal.