An optical proximity sensor with a digital calibration circuit and a digital calibration method are provided. In a test mode, a test light emitted by a light transmitter is reflected by a cover of an electronic device to form a first reflected analog signal to a light receiver. A digital calibration circuit counts the first number of pulse waves of a first reflected digital signal. In a calibration mode, a light is emitted to a detected object through the cover and then is reflected by the detected object to form a second reflected analog signal to the light receiver. When the digital calibration circuit counts the number of pulse waves of a second reflected digital signal up to the first number, the digital calibration circuit clears the first number and then recounts the number of pulse waves of the second reflected digital signal to obtain the second number.