Digital instrumentation and control (I&C) systems are widely used in many industrial areas. In the recent years, the digitalization process for nuclear power plants has also been moving on rapidly. Full digitalized I&C systems are now adopted in almost all new constructed nuclear power plants. Moreover, for the advanced small modular reactors, both the high reliability and extendibility of I&C systems are especially required. The original architecture, which usually consists of centralized control cabinets and severs, of I&C systems cannot adequately provide the required extendibility. A novel hardware architecture of distributed control system aiming to enhance the reliability and extensibility was proposed and presented in the 26th International Conference on Nuclear Engineering (ICONE26). In this paper, we further focus on the considerations about the software design of this novel decentralized architecture. The hardware entities and their characters related to the software design are first introduced. Then the key considerations, including the design objective and software framework are described. Moreover, the related algorithms regarding the coordinate node selection and task assignment are proposed and demonstrated. At last the analysis about the software for the decentralized architecture are provided. The previously proposed decentralized architecture becomes feasible based on the software design introduced in this paper.