Trip of the security and stability control equipment directly affects the safe operation of power systems， and the correct judgment to trip an AC transmission line offers an essential prerequisite for the correct trip of the SSC. This paper studies the characteristics of electric changes if the transmission line is tripped， and the differences of line trip from the characteristics in the dynamic process， e.g. power transfer， out-of-step， lower-frequency oscillation， etc. Based on the accurate implication of the current trip criterion and relevant assessing conditions， the paper also analyses the reasons of the mal-trips of SSC and puts forward effective measures of prohibiting the mal-trips， so as to upgrade the reliability of SSC.