Since the 1950s, scramjet has been regarded as a goal of the hypersonic propulsion system in the aerospace field, and extensive research work has been conducted in both theory and practical technology. At the beginning of the 21st century, the United States made a series of technological breakthroughs close to practical applications, pushing the research and application of the scramjet technology to a new stage. This paper briefly describes the working principle of the scramjet, discusses the difficulties and latest progress in key technologies such as dual-mode working process, hypersonic compression flow, supersonic combustion, ultra-high temperature structure and thermal protection, ground test and numerical simulation, and expounds the understanding and suggestions on the development direction of key technologies such as wide speed range, repeatability, and higher Mach number of scramjet.