Featured with long distance, large capacity, extra high voltage and AC-DC interconnected operation, China Southern Power Grid is interconnected with surrounding areas and countries through multiple AC lines, and has complex operation characteristics. In combination with the operation practice of Southern China regional power system during recent years, the operation characteristics of complex AC-DC interconnected power grid is discussed in depth. This paper reveals the main security risks in the operation of Southern China regional power system, including: the prominent risk of single-cause common mode fault, the prominent stability problem caused by the primary and secondary equipment failure in the AC-DC close coupling scenario; and the security of main power grid caused by the stability problems of frequency and rotor angle seriously. On the basis of summarizing the stability control methods, this paper proposes to build the power system security and stability defense lines based on wide-area information, which will make the China Southern Power Grid more secure and stable in the future.