1. An image reconstructing method using X-ray volume photography, comprising the following steps: x-ray camera acquiring data of a plurality of X-ray area array images by taking photos of an object interested according to the locating principle of the digital photographic measurement, when taking an X-ray digital photo, simultaneously taking at least three mark points of which the 3D positions are known in the X-ray digital photo, wherein the at least three mark points are disposed on a plane mark frame; a computer using the coordinates of the mark points in a rectangular coordinate system for volume photographic 3D image reconstruction and the coordinates of the mark points in an X-ray photo coordinate system o-xy to construct a transforming relationship of the two coordinate systems, determining the path and the position of each X-ray photographic beam by using the transforming relationship, obtaining all voxels of each X-ray passing the object interested, determining the gray scale value of the voxel g by solving the Radon equation set based on a voxel g and a pixel G on the same X beam path, calculating the gray scale values of all voxels g, reconstructing a 3D volume image of the object interested based on the gray scale values of the voxels g; wherein the X-ray photo coordinate system o-xy is constructed by the following: constructing a photo coordinate system o-xy on the X-ray digital photo, the origin o of the photo coordinate system being a pixel of the vertical beam of an X-ray tube center S on the X-ray digital photo, x axis of the photo coordinate system being the lateral arrangement direction of the pixels passing through the origin o of the photo coordinate system, y axis of the photo coordinate system being the vertical arrangement direction of the pixels passing through the origin o of the photo coordinate system; wherein the rectangular coordinate system O-XYZ for volume photographic 3D image reconstruction is constructed by the following: disposing at least three mark points on the plane mark frame for the object interested and measuring the 3D distances between every two of the at least three mark points, taking an arbitrary mark point as the origin O of the coordinate system to construct the rectangular coordinate system O-XYZ for volume photographic 3D image reconstruction, storing the coordinates of each mark point in the coordinate system O-XYZ into the computer.