Fiber supercapacitor (FSC) is a promising power source for wearable/stretchable electronics and high capacitive performance of FSCs is highly desirable for practice flexible applications.Here,we report a composite of manganese dioxide (MnO2) and activated carbon fibers (ACFs) with high MnO2 mass loading and microporous structure (abbreviated as MnO2@ACF),which is used as a fiber electrode to produce a FSC with a high capacitive performance and a good flexibility.The MnO2@ACF composite electrode in FSCs delivers an ultrahigh specific capacitance of 410 mF/cm2 at 0.1 mA/cm2,corresponding to a high energy density of 36 μWh/cm2 and high power density of 726 μW/cm2.Such high capacitive performance and simple fabrication method indicates that the MnO2@ACF composite is a very promising electrode material for flexible fiber supercapacitors.