Aiming at the problem that the decoding algorithm of encoding target is complicated and the decoding accuracy is easily affected by the shooting angle in the field of photogrammetry, a encoding target and its decoding method based on the principle of projective invariance are designed. The template of the coded target includes a reference circle and several coded circles, 4 coded circles and reference circles are selected from the template to form the target, and the coding function is realized by selecting different coded circle combinations. In the decoding process, the geometric position and area relationship between the reference circle and the code circle are used to realize the rapid and automatic positioning of the reference circle and the code circle. By calculating the projective invariant of the intersection ratio of the center of five circles in the target, the fast automatic decoding of the target is realized. The experimental results show that the decoding accuracy of the designed target is higher than that of the traditional coding target when the shooting angle is large, because the projective invariant is used as the encoding information, and the decoding accuracy of the designed target can reach 100% when the shooting angle is 60°.