Abstract: Power electronic transformer（PET） which is also called solid-state transformer（SST） or smart transformer is a new electric power conversion equipment that can fulfill the function of voltage transformation, galvanic isolation, power flow control and regulation, renewable energy interfacing and so forth. The research and development of PET technologies has gained more and more attention with the fast development of future grid technologies, such as smart grid, the energy internet etc. Whereas, there are still many technical issues to be resolved before large-scale applications. Based on the PET development history, this paper summarizes and sorts out the PET related key technologies, especially the circuit topology, control and protection, high-frequency transformer design and optimization, compact design and optimization of the power circuits, application of wide bandgap power semiconductors and so on. In the end, the key obstacles to the further development of PETs and the technical trends are also summarized.