Fourier ptychographic microscopy (FPM) is a recently developed computational microscopy. Different from the conventional microscopy, the FPM generally employs a programmable LED array as an illumination source for angular illuminations. The intensity of LED illumination is corresponding to the emitting angle. The luminous intensity decreases rapidly with the increasing of emitting angle. The illumination intensity of different angles cannot be consistent, which leads to the decline of the reconstructed image quality. Consequently, the intensity correction of low-resolution raw images is consequently needed before the iterative phase retrieval process. The reasons of the different angles of illumination intensity inhomogeneity are introduced. Numerical simulations demonstrate the necessity of the intensity correction. Finally, the experimental results of physical intensity correction are given.