The vignetting caused by the external occulter and support pylon of externally occulted white light coronagraph is analyzed and calculated. The coronagraph is with a field of view of 2.5~15 R⊙ (Sun Radius), a resolution of 14 sec/pixel, an aperture of 30 mm and an effective focal length of 200 mm. An analytical expression of the vignetting as a function of the distance between external occulter and objective is obtained. The vignetting figures at different distances are acquired by simulation. It is shown that the longer the distance is, the smaller the vignetting. The theoretical results are verified by actual experiments with proposed prototype of the coronagraph. The coronagraph image of target is corrected by vignetting compensation, and the image quality is improved greatly.