The concept of a digital twin of an automated process control system (APCS) of a thermal power plants (TPPs) is considered. The principles of its formation and use are proposed on the basis of a design database developed within the framework of a computer-aided design (CAD) system for thermal automation and measurements. Examples of design automation systems aimed at solving similar problems are given and the features of the proposed CAD system are indicated. The typical structure of the APCS and the relationship of its main components—sensors, cables, executive and switchboard devices, signal connection tables, as well as the software and hardware complex (composition and distribution of modules of communication devices with an object)—are described. A description of the project database’s structure is given, which provides comprehensive work with both project documentation and information about the main components of the APCS. The sequence in execution of design procedures and links between the design database and the CAD database during their implementation are shown. Methods for increasing the productivity of designers are considered, such as the use of standard solutions in the organization of measurements and control schemes for actuators of thermal power plants, multiple use of information with a single input, and synthesis of text and graphic design documentation based on the project database. It is proposed to use a digital twin at all stages of the object’s life cycle: during design, installation, commissioning, and operation. The effectiveness of working with information thanks to an intuitive interface for navigating through the project was demonstrated. Examples of objects, in the implementation of which the considered CAD system was used, are given.