A polarization point diffraction interferometry (PPDI) system, adopting a specially designed even aspheric quarter-wave plate (EAQWP) in the test path, is proposed for low-reflectivity and high-numerical-aperture spherical surface testing. In terms of the low-reflectivity mirror measurement, the obtained poor fringes contrast, which can significantly affect the measurement accuracy, can be improved by the polarization characteristic of the EAQWP. Simultaneously, the wavefront distortion, especially larger in high-numerical apperture (NA) measurement, can be greatly reduced attributable to the even aspheric surface design instead of the plane. In addition, the pose error introduced by EAQWP is demonstrated in detail, and a difference restoration model is built to calibrate it. Consequently, the location of the EAQWP is fixed in the test path after calibrating the pose error, having no need to be adjusted with the change of the spherical mirror under test, which facilitates the system alignment in practical optical shop testing. Ultimately, the surface error of a spherical mirror with a low-reflectivity of 0.04 and a high NA of 0.5 is measured with PPDI. The experimental results are validated to be in good agreement with that of a ZYGO interferometer.