Based on the 2011 Southwest America blackout inquiry report， this paper presents a description of the event evolution and an analysis of technical causes leading to the cascade development from the point of view of prevention and control of cascading outages， including ineffectiveness of security analysis and preventive control， inadequacy of real-time situational awareness， unreasonability of protection relay settings， inappropriateness of control system design and lack of coordination between protection and control. It concludes that several fundamental measures need to be strengthened， including a complete， thorough and timely understanding of system dynamic characteristics， coordination of protection and control actions， and improvement of simulation analysis.