Acording to Fourier optics, when a plane wave through a grating,the angle of different order and relative intensity are decided by the grating, many information of gratings can be deduced from its diffraction power spectrum. A new method using power spectrum of different orders to measure depth of phase gratings.three gratings were tested in this paper and the calculated depth was givem. The testing results of diffractive order of 6 gratings-about many orders itensity-were listed in table, The error of results is under 5%.