Rechargeable sodium (Na) metal batteries (SMBs) provide an alternative energy-dense, low-cost energy storage system beyond prevailing Li-ion technologies. However, their practical deployment requires the performance evaluation of Na anodes on the device level and mitigation of safety hazards, i.e., mechanical stress, electrode pulverization and dynamic interfacial properties. Here, we propose a lightweight N-doped carbon nanofiber substrate with SnSb nanocrystallites monodispersed within the mesopores (SnSb@NCNF). The alloying-induced Na15Sn4 and Na3Sb intermediates act as the heterogeneous sodiophilic "magnets" to homogenize Na ion flux and confine Na deposits within the nanospace. Additionally, the nucleation theory of metal solidification bridges the density functional theory calculations, elucidating the oriented Na propagation that maximizes the anode utilization. With the proper Na plating regulation, the SnSb@NCNF substrate (pre-stored 1 x excess Na) integrates with the NaVPO4F cathode in 5 mA h single-layer pouch cell, the prototype of which exhibits the cycling endurance (96.3% capacity retention for 500 cycles) and high specific energy/power densities even upon the repetitive mechanical flexing scenarios. The nanoconfinement of the heterogeneous nucleation process affords a feasible approach to mitigate the dendrite formation upon the geometry deformation or high areal-capacity loading, which enlightens the further exploration of the energy-dense, mechanical flexible battery system.