Harvesting fog composed of differently charged droplets offers a potential solution to freshwater crises. Leveraging electrostatic attraction between charged surfaces and droplets to enhance capture efficiency represents an efficacious approach for achieving efficient fog harvesting. However, existing strategies to enhance electrostatic attraction by introducing charges on the surface pose persistence challenges. Here, an asymmetric wettability polyacrylonitrile (PAN) fiber (named Janus-PAN) with stable high surface potential via in-situ molecular confined modification is proposed for fog harvesting. By coupling the high capture efficiency generated by persistent electrostatic interaction and the directional self-driven transport supported by wettability gradient, Janus-PAN achieves a water collection rate (WCR) of 1775 mg/cm2/h, which is 2.6 times higher than that of fibers with low surface potential and no wetting gradient. Moreover, the potential application of the Janus-PAN harp in agricultural irrigation is demonstrated. The previously unreported surface potential control strategy shown here can potentially upgrade the fiber-based fog harvesting materials.