As an emerging research hotspot with the background of "Industry 4.0", digital twin provides key technical means to support the challenges to relay protection brought by the new power system. For this reason, digital twin is analyzed, and combined with the practical application requirements in the field of relay protection. The relevant terms of relay protection digital twin are defined, and its expected characteristics are summarized. A typical architecture of relay protection digital twin system is proposed. In view of the shortcomings of existing research on relay protection digital twin in terms of data real-time interaction ability, protection logic transparency, interface standardization, etc., the key technologies for its construction are noted. Finally, the potential application value of the technology is explored. It is expected to provide some reference for further research and application of digital twin technology in this field.