Spherical surfaces are essential components in optical systems for different applications. Also, standard spheres are typically employed to calibrate dimensional and mass-measuring instruments. Therefore, measuring and eliminating the surface irregularities of these spheres represents a challenge for modern industries. Surface irregularities can be measured by either the contact method (as stylus profilometers) or the non-contact method (as interferometers). The Fizeau laser interferometer (GPI-XP, Zygo Co.) is typically used to accurately measure surface irregularities for flat surfaces, lenses, and mirrors of 102 mm diameter, and it is not prepared for testing heavy spheres. This work presents a modification for the interferometric system to expand its capability for testing spheres of diameters up to 145 mm and masses up to 2 kg. Also, the modification provides automatic rotation for the tested surfaces (an electric rotating platform) to avoid the disadvantages of manual rotation (a tweezer). At different rotation angles, the measurements are performed on a mass-standard silicon sphere (SiScKg-02-d, PTB Inst.). The average measured value of the surface irregularities is 43.9 nm, with an improved repeatability of 2.8 nm and a minimized measurement uncertainty of ±6.8 nm. The evaluation of the uncertainty budget is also investigated in this study.