1. A method for correcting image distortion in diffusion-weighted echo planar imaging, comprising: utilizing a combined sequence for acquisition to obtain a plurality of marked images with different preset b values and different preset diffusion directions, wherein the combined sequence comprises a diffusion-weighted echo planar imaging sequence and a marker sequence applied before the same; and utilizing the diffusion-weighted echo planar imaging sequence for acquisition to obtain a plurality of diffusion-weighted echo planar images with the same b values and diffusion directions as the marked images; subtracting each of the marked images from the diffusion-weighted echo planar image having the same b values and the same diffusion directions to obtain a marker image corresponding to each of the diffusion-weighted echo planar images, wherein a marker image with the b value being zero is a reference marker image, and a marker image with the b value not being zero is a distortion marker image; and for each diffusion-weighted echo planar image with the b value not being zero, utilizing the distortion marker image having the same b value and the same diffusion direction as the diffusion-weighted echo planar image and a reference marker image corresponding to the diffusion-weighted echo planar image to calculate a stretching coefficient and a displacement coefficient corresponding to each image data column of the diffusion-weighted echo planar image, and utilizing the stretching coefficient and displacement coefficient to correct the diffusion-weighted echo planar image.