Geometric algorithms are widely used in many scientific fields like computer vision, computer graphics. To guarantee the correctness of these algorithms, it's important to apply formal method to them.We propose an approach to proving the correctness of geometric algorithms. The main contribution of the paper is that a set of proof decomposition rules is proposed which can help improve the automation of the proof of geometric algorithms. We choose TLA+2, a structural specification and proof language, as our experiment environment. The case study on a classical convex hull algorithm shows the usability of the method.