Abstract: A phase-detection device and method for an optical element are provided, along the direction of the optical path the phase-detection device includes: a light source, a beam collimator, a diffraction element and an imaging detector; the optical element is set between the filter and the diffraction element; the diffraction element includes a mesh mask region and an array region segmented by the mesh mask region; mesh mask region blocks lights of a target wavelength; the array region includes a first class of cell and a second class of cell, and the first class of cell and the second class of cell are alternatively arranged; the first class of cell provides a first phase for the lights of target wavelength, and the second class of cell provides a second phase for lights of target wavelength; a phase difference between the first phase and second phase is T rad.