In accordance with that setting value of a voltage element is equal to bus voltage on installation location of protection during phase-to-phase fault on the protection coverage end， based on the node impedance matrix of positive sequence network and general computation model for short circuit fault on the power line， a new method is proposed to directly obtain the protection coverage of voltage element by solving a unary quadratic equation. This method is suitable for all kinds of complex power network structures with such advantages as general-purpose， less computation burden， accurate results and easy to program， etc.