Aero-engine design and simulation technologies thrive in metasynthesis and integration. Meanwhile, with the improvement of system engineering methodology, the digital revolution in aero-engine field is in the ascendant. The concept, current situation, demand and key technical issues of aero-engine digitization are reviewed in the hope of providing reference and support for chinese aero-engine digital transformation. Facing the complex essence of aero-engine, the concept and connotation of model-based system engineering, digital twin and digital thread are analyzed, and the definition of digital engine is further developed. The use cases and development status of digital engine at home and abroad are summarized and the work needing to be done including model and algorithm library, general platform, data connectivity and processing, coupled and integrated simulation technologies, data analysis and decision making is discussed, according to the technical trends and challenges. Aiming at the key technical problems of digital transformation, the relevant research content and direction of the disciplinary simulation integration method, the communication method of system model and discipline model, and the digital twin modeling method are discussed. The idea and specific research content of digital engine can provide the foundation for subsequent planning and construction of aero-engine digital twin systems.