Since traditional controlled islanding analysis method does not consider reactive power support，it often results in the unacceptable power flows in the islands，even if the constraint of generatorscoherency and active power balance are satisfied.The main reason is the inappropriate structure of the islands.It causes abnormally low bus voltages or even divergence of power flow calculation.In order to improve the voltage profiles of an islanded system，a voltage controllability constraint is used to assist the controlled islanding process.The controlled islanding is operated by a new efficient clustering method.In the method，the voltage controllability constraint is defined by VQ sensitivity and reactive power capacity.In addition，a new electrical distance expression which combines the active power balance and the voltage controllability is proposed.Finally，an improved K-Medoids clustering algorithm is used to obtain the controlled islanding scheme.Case studies and results comparisons show the effectiveness of the proposed method.