Abstract: The digital reactor is an integral numerical simulation platform for the performance of nuclear reactor systems. In the first part of this paper, the development history of the nuclear reactor simulation technology is reviewed. The three technical elements constituting the digital reactor are elaborated, including the target scenario, advanced models and multi-physics coupling technology, and the integrating environment. Although there are several challenges for the development of digital reactors, such as the difficulties in multi-physics and multi-scale computation, the complexity in design optimization, and the insufficient database, the digital reactor can help better analyze key problems that limiting the reactor performances and safety, and better understand the mechanism of  the phenomena that cannot be observed or measured experimentally.