Most metal-organic frameworks (MOFs) hardly maintain their physical and chemical properties after exposure to alkaline aqueous solutions,thus precluding their use as potential electrode materials for electrochemical energy storage devices.Here,we present the design and synthesis of a highly alkaline-stable metal oxide@MOF composite,Co3O4 nanocube@Co-MOF (Co3O4@Co-MOF),via a controllable and facile one-pot hydrothermal method under highly alkaline conditions.The obtained composite possesses exceptional alkaline stability,retaining its original structure in 3.0 M KOH for at least 15 days.Benefitting from the exceptional alkaline stability,unique structure,and larger surface area,the Co3O4@Co-MOF composite shows a specific capacitance as high as 1020 F g-1 at 0.5 A g--1 and a high cycling stability with only 3.3％ decay after 5000 cycles at 5 A g-1 The as-constructed solid-state flexible device exhibits a maximum energy density of 21.6 mWh cm-3.