1. A method for virtual viewpoint synthesis in a multi-viewpoint video, comprising: the step 1 of performing three-dimension image transformation on a first reference viewpoint image obtained by a first camera according to the first reference viewpoint image and its depth image, and parameters of the first camera, to obtain a first virtual viewpoint image and its depth image and hole map; and performing three-dimension image transformation on a second reference viewpoint image obtained by a second camera according to the second reference viewpoint image and its depth image, and parameters of the second camera, to obtain a second virtual viewpoint image and its depth image and hole map; the step 2 of filling small holes of the first virtual viewpoint image through horizontal, vertical and diagonal extrapolations according to the hole map of the first virtual viewpoint image, to obtain the small holes-filled first virtual viewpoint image; and filling small holes of the second virtual viewpoint image through horizontal, vertical and diagonal extrapolations according to the hole map of the second virtual viewpoint image, to obtain the small holes-filled second virtual viewpoint image; the step 3 of performing image complementing by taking the small holes-filled first virtual viewpoint image and second virtual viewpoint image as reference images of each other, to obtain the image-complemented first virtual viewpoint image and second virtual viewpoint image; the step 4 of performing fusion processing on the image-complemented first virtual viewpoint image and second virtual viewpoint image, to obtain a primary virtual viewpoint image; the step 5 of performing depth image processing respectively on the first reference viewpoint image and the second reference viewpoint image, to obtain the depth image-processed first reference viewpoint image and second reference viewpoint image; and then performing the corresponding processes on the depth image-processed first reference viewpoint image and second reference viewpoint image according to the step 1 to the step 4 , to obtain a secondary virtual viewpoint image; and the step 6 of filling holes of the primary virtual viewpoint image according to the secondary virtual viewpoint image to generate a virtual viewpoint image.