It is an important task verifying the phase sequence of newly built power transmission lines prior to ring closing and going into operation.Despite the low probability of transmission lines ring closing with a wrong phase sequence，it does happen once in a while and bring definite harm to power system safe and stable operation.A method based on the voltage and current phasor diagram is proposed to analyze the special form of fault of ring closing with a wrong phase sequence.The characteristics of ring closing with a wrong phase sequence are different from those of normal operation or short circuit fault.And the method is proved correct through RTDS simulation.Finally，the operation of protective relays is discussed according to the specific models and principles of protective relays under the fault of transmission lines ring closing with a wrong phase sequence.