Determining security/stability boundaries is a common and critical means of preventing cascading failures induced by voltage-related issues, which represents one of the major challenges in bulk power systems. However, traditional approaches suffer from conservative issues and heavy computational burdens. To address these challenges, the concept of an autonomous-synergic voltage security region (AS-VSR) and the corresponding dynamic constraint coefficient pruning (DCCP) computation method, which fully consider the volt/var characteristics of bulk power systems, are proposed in this letter. Both linearized and nonlinearized robust optimization problems are introduced to obtain accurate results. The computational accuracy, time cost, and advantages of autonomous-synergic control are observed in the simulation results.