Abstract: The strategy of blocking inter-turn protection is adopted when potential transformer（PT） secondary circuit is open, but it can expand the failure range easily. This paper analyzed the calculation method of zero sequence impedance when reactor PT secondary circuit open, characteristics of initial and terminal current was studied after inter-turn fault and external fault at the reactor, new criterion about shunt reactor inter-turn protection when PT secondary circuit open was proposed on the basis of zero sequence impedance and fault current characteristic, the correctness of the theoretical analysis was verified in PSCAD/EMTDC electromagnetic transient simulation platform, this criterion improves the existing reactor protection methods. Reactor and line system model was established in the dynamic simulation lab, dynamic simulations results verified the validity of the theoretical analysis and calculation.