The present invention relates to an apparatus for accurately detecting a mains phase. The apparatus is constructed with a zero-crossing detector, a digital phase detector, a digital loop filter, and a digital controlled oscillator (DCO) of a direct digital synthesized (DDS) manner. The present apparatus employs an all-digital loop architecture and a high sampling clock to recover a signal with a phase orthogonal with the mains signal and a frequency the same as the mains signal. And jitters in the recovered signal are less than 10 us. The present apparatus is capable of implementing signal tracking of a zero frequency error and a zero phase in a wide range, and can provide a detection result of excellent performance for the power line carrier communication, mains frequency detection, etc.