To measure spherical surface with high precision, an advanced fiber point diffraction interferometer (FPDI) is introduced in this paper. Based on the system principle, the factors that affect the interference image contrast are put forward and analyzed, such as fiber optical properties, interferogram background and intensity match of two beams. Then the optimization method is proposed. The single-mode fiber with core diameter 3λ is selected and the dispersion between modes is removed. The short coherence length laser is designed with a semiconductor solid-state laser and meets the need of FPDI. By coating film on the fiber end face, the light use ratio is improved significantly. Finally, the fiber point diffraction interferometer system is set up in experiment and the interference fringes are obtained. The result shows that the FPDI can achieve interference patterns with high contrast after optimization and the system has reached a relatively high accuracy.