With the development of interconnected scale of power grid, the foundation to realize large scale resource optimization configuration is established, and power system stability problems become complicated. More comprehensive control measures for large scale power grid security and stability control will be supported by wide area measurement system (WAMS). The mechanism of power system transient stability improved by emergency DC power control (EDCPC) is introduced based on extended equal area criterion (EEAC) theory. In AC/DC parallel connection grid, the imbalanced power induced by EDCPC will cause an extra feedback which might make adverse effects on some unstable mode through AC transmission lines. As a result there exit coordinated space between EDCPC measures and AC control measures, such as generators tripping, load tripping and so on. To evaluate the effects of AC/DC coordination control, a coordinated and optimal mathematical model is built. It is necessary for AC/DC coordination control which is verified by simulation.