This paper proposes a new snapshot method for continuous data protection (CDP) system that considers the disability of taking large amount of snapshots in traditional CDP systems. The snapshot method (Convex Point SNAPshot, CSNAP) is based on the concept of convex point set. After the data structure of CSNAP and introduced the concept of convex point based on the pointers in the data structure have been discussed, the study analyzes the properties of convex point set and proposed CSNAP algorithms. An enhanced CSNAP method by introducing the concept of retro-cost is also proposed. Finally, the study uses a typical workload and random generated trace data to test CSNAP method. The experimental results show that at average CSNAP takes less than 10% storage space of traditional snapshot method.