1. A 3D image-capturing method, comprising: using a left-eye camera to capture a left-eye image and using a right-eye camera to capture a right-eye image; comparing the left- and right-eye images to observe similar columns between the left- and right-eye images and thereby obtain a first-column range in the left-eye image and a second-column range in the right-eye image which indicate similar columns; comparing the left-eye image within the first-column range with the right-eye image within the second-column range to observe similar rows between the left- and right-eye images and thereby obtain a first-row range in the left-eye image and a second-row range in the right-eye image which indicate similar rows; and vertically shifting the left- and right-eye images in accordance with the first-row range obtained in the left-eye image and the second-row range obtained in the right-eye image to horizontally align the left- and right-eye images to generate a 3D image, wherein the step of observing the first-column range in the left-eye image and the second-column range in the right-eye image further comprises: estimating vertical projections of the left- and right-eye images; differentiating the vertical projections of the left- and right-eye images to obtain first-order differentiated vertical projections of the left- and right-eye images; and shifting the first-order differentiated vertical projections of the left- and right-eye images relative to each other step by step with comparisons between each shift and thereby the first-column range in the left-eye image and the second-column range in the right-eye image are obtained.