1. A three-dimensional magnetic resonance imaging method, comprising: applying a slab selection gradient to a subject by a gradient output module; transmitting a radiofrequency pulse to the subject by a RF excitation module, and exciting a slab of the subject to produce magnetic resonance signals with a continuous frequency bandwidth; performing a spatial encoding gradient across three dimensions by the gradient output module to encode the magnetic resonance signals, wherein an equivalent encoded field of view which along a selected acceleration direction is controlled by the spatial encoding gradient, and the equivalent encoded field of view of that direction is made shorter than that of a normal 3D MRI scan; applying a separation gradient G sep along with the spatial encoding gradients G spen by the gradient output module to produce encoded resonance signals; and receiving the encoded magnetic resonance signals by the RF receiving module, and reconstructing the encoded magnetic resonance signals into 3D images by an imaging processing module.