After uncovering the negative effects of angle stability control in 1997， negative effects of load shedding control on transient frequency and voltage security were discovered. Based on EEAC and the quantitative analysis theory of transient frequency and voltage security， the mechanism of the newly uncovered negative effects is deeply investigated. The complexity of power system stability is explored， and the robustness of optimal load shedding control is improved. A three-machine nine-bus case which is easy to reproduce is given， and also the cases of practical power systems are given.