Unidirectional conduction block of cardiac excitation wave is one of the necessary conditions leading to initiation of reentrant arrhythmia. Temporal vulnerable window (VWtime) is an important measure for arrhythmia vulnerability. In this study, we examine theoretically how low extracellular K concentration ([K]o) control the VWtime by a single premature extrasystole. A numerical modified LR91 one dimensional heterogeneous ventricular model is used to quantitatively investigate the relation of low [K]o with conduction velocity (CV), VWtime, and electrical dynamic factor CV restitution. Morever, we characterize the distribution of Na channel conductance (gNa) and gating factors of premature beat applied at different time of the VWtime with different [K]o. The results show that lowering [K]o enlarge the VWtime magnitude gradually. The CV of S1 beat slows down with the [K]o in a reverse-[K]o dependent manner. Low [K]o promote the recovery of maximum Na conductance (GNa) from inactivation. However, decreasing [K]o can increase heterogeneity of gNa along the cable, and modulate CV restitution curve, which induce larger transmural dispersion of refratoriness for the premature beat and enlarge the VWtime.