In order to realize a large field, high resolution, and dynamic measurement of phase defects of optical components, a Reflective Shearing Point Diffraction Interferometer (RSPDI) was proposed. A lateral shear amount was introduced between the test and reference light to generate a high-density linear carrier frequency, and the information of the wavefront under the test was extracted from a single interferogram by an fast Fourier transform algorithm to determine the simultaneous measurement of the defects. The lensless imaging algorithm suppressed the diffraction effect of the defect, and the effective discrimination method of the defect type was summarized. In the experiment, an optical flat in a high-power laser system was detected by the RSPDI, and the correctness of the defect type criterion was verified. In addition, a laser-damaged optical plate was measured by the RSPDI and a Veeco NT9100 white light interferometer, with a relative error of 2.1%. The results indicate that the interferometer can be used to detect phase defects of large-aperture optical components effectively.