Fourier ptychographic microscopy (FPM) is a newly developed imaging technique which stands out by virtue of its high-resolution and wide FOV. It improves a microscope's imaging perfor-mance beyond the diffraction limit of the employed optical components by illuminating the sample with oblique waves of different incident angles, similar to the concept of synthetic aperture. We propose to use an objective lens with high-NA to generate oblique illuminating waves in FPM. We demonstrate utilizing an objective lens with higher NA to illuminate the sample leads to better resolution by simulations, in which a resolution of 0.28 m is achieved by using a high-NA illuminating objective lens (NA=1.49) and a low-NA collecting objective lens (NA=0.2) in coherent imaging (λ=488 nm). We then deeply study FPM's exact relevance of convergence speed to spatial spectrum overlap in frequency domain. The simulation results show that an overlap of about 60% is the optimal choice to acquire a high-quality recovery (520*520 pixels) with about 2 min's computing time. In addition, we testify the robustness of the algorithm of FPM to additive noises and its suitability for phase objects, which further proves FPM's potential application in biomedical imaging.