Improving the energy storage capabilities of aqueous alkaline supercapatteries relies heavily on the development of high-performance anode materials. Here, we presented a one-step composite molten salts method for the preparation of cobalt selenium hydroxide and BiOSe heterostructure composites (CoSe(OH)/BiOSe). The composite capitalizes on the advantageous interlayer properties of two-dimensional (2D) layered BiOSe, connected via weak Van der Waals forces, alongside the high theoretical specific capacity of cobalt hydroxide derived from cobalt selenium hydroxide. This synergy facilitates efficient ion transport, thus exhibiting remarkable electrochemical characteristics. Specifically, CoSe(OH)/BiOSe demonstrates a great specific capacity of 313.6 mAh/g at 1 A/g, significantly outperforming pure BiOSe nanosheets. Moreover, CoSe(OH)/BiOSe exhibits remarkable rate capability with 63.8% capacitance retention at sufficiently high current density (20 A/g), and excellent cycle performance that the capacitance remains is 70.3% after 1000 cycles of GCD testing. Additionally, ex-situ XRD analysis of CoSe(OH)/BiOSe at different charge and discharge stages provides further insights into its energy storage mechanism. Furthermore, the NiAl-Cl LDH// CoSe(OH)/BiOSe device demonstrates high power density and energy density. These results validate the feasibility of enhancing the electrochemical performance of 2D BiOSe layered materials through the incorporation of active and cost-effective transition metal oxides/hydroxides, highlighting the tremendous potential of CoSe(OH)/BiOSe as a negative electrode material for aqueous alkaline supercapattery.