Metal-organic framework (MOF)/polymer composites have attracted extensive attention in the recent years. However, it still remains challenging to efficiently and effectively fabricate these composite materials. In this study, we propose a facile one-pot electrospinning strategy for preparation of HKUST-1/polyacrylonitrile (PAN) nanofibrous membranes from a homogeneous stock solution containing HKUST-1 precursors and PAN. MOF crystallization and polymer solidification occur simultaneously during the electrospinning process, thus avoiding the issues of aggregation and troublesome multistep fabrication of the conventional approach. The obtained HKUST-1/PAN electrospun membranes show uniform MOF distribution throughout the nanofibers and yield good mechanical properties. The membranes are used as separators in Li-metal full batteries under harsh testing conditions, using an ultrathin Li-metal anode, a high mass loading cathode, and the HKUST-1/PAN nanofibrous separator. The results demonstrate significantly improved cycling performance (capacity retention of 83.1% after 200 cycles) under a low negative to positive capacity ratio (N/P ratio of 1.86). The improvement can be attributed to an enhanced wettability of the separator towards electrolyte stemmed from the nanofibrous structure, and a uniform lithium ion flux stabilized by the open metal sites of uniformly distributed HKUST-1 particles in the membrane during cycling.