The demand for the application of the interlocking logic in the substation automation based on wide area security defense is analyzed， and a multi-layer logical interlocking mechanism in the substation automation consisting of the bay layer， station layer and wide area protection layer is proposed. The method of assembling the dynamic information in a power system using phasor measurement units（PMU） belonging to wide area measurement system（WAMS） is presented. An on-line security restriction checking mechanism of the wide area protection layer is designed， the implementation techniques of this mechanism are described， and an application example is given.