A backward depth mapping method for stereoscopic image synthesis includes: providing a first coordinate and a first depth value of a pixel of a virtual view in a stereoscopic image; calculating a second coordinate and a second depth value of a pixel of reference view, which is obtained by mapping the virtual view, based on the first coordinate and the first depth value of the pixel of the virtual view; calculating an error value corresponding to the first depth value based on the second coordinate and second depth value; and determining a third depth value of the pixel of the virtual view based on the error value. As a result, it is possible to avoid the virtual view producing a crack and the pixel of the background from appearing at the crack of the foreground.