Flexible electrode materials show many advantages and hold great prospects for energy storage application. But, the synthesis processes of these kind of materials are always complicated, are low in efficiency and high in cost. Here, we propose a facile and cost-effective two-step synthesis strategy of a flexible electrode by growing ultrathin and vertical CoNiS nanosheets on nitrogen-doped carbon foam (CoNiS NSs@NCF). The NCF is obtained by direct carbonization of the melamine foam. When evaluated as binder-free electrode material for supercapacitor in three-electrode system, the CoNiS NSs@NCF exhibits an excellent specific capacitance of 1576.8 F g and a superior cycling stability (91.5% capacitance retention at the 5000th cycle). Then, an asymmetrical supercapacitor was fabricated using the as-synthesized material as the positive electrode and activated carbon as the negative electrode, which delivers a high energy density of 42.8 Wh kg at a power density of 399.7 W kg, remarkable rate capability and satisfactory cycling stability (85.3% capacitance retention at the 5000th cycle). In brief, our work offers a low-cost and facile approach to prepare promising flexible electrode materials for high-performance supercapacitors.