A circuit and a method for automatically calibrating a phase interpolator are provided. Phase information of a reference clock signal and an output clock signal are processed by a phase detector to detect a phase difference of the two clock signals. A difference value between the phase difference and a standard phase difference corresponding to the digital control code is obtained, to generate compensation information. The compensation information is sent to the phase interpolator control unit for storage. When the phase interpolator operates normally, a phase interpolator control unit generates a control signal based on the compensation information, to regulate the phase value of the output clock signal of the phase interpolator.