An auto phase detection apparatus for automatically detecting a target sampling phase is provided. The auto phase detection apparatus includes a phase decider for generating a plurality of phase control signals; a sample clock generator, coupled to the phase decider, for generating a plurality of sample clock signals according to the phase control signals; an analog-to-digital converter (ADC), coupled to the sample clock generator, for converting an analog video signal to a digital signal according to the sample clock signals; a phase detector, coupled to the ADC and the phase decider, for generating a plurality of phase detection results according to the digital signal; and a motion detector, coupled to the ADC and the phase decider, for generating a motion detection result by detecting a motion in the digital signal. The phase decider determines the target sampling phase from the phase control signals according to the phase detection results and the motion detection result.