Mixed-valence is an effective way to achieve high electrochemical performance of anodes for supercapacitor. However, inordinate mixed valence with more structural defects leads to structural instability. The development of mixed valence electrodes that can maintain a stable structure during the defect formation process is the key to resolving this problem. CuSe with mixed-valence is a potential candidate, the stable monoclinic structure of Cu_2-xSe can be transformed into another stable cubic structure (x > 0.15). Herein, Cu_1.85Se anode with mixed valence reveals the ultrahigh specific capacity of 247.8 mA·h/g at 2 A/g. Furthermore, the introduction of multi-walled carbon nanotubes (MWCNTs) into Cu_1.85Se further improves the specific capacity (435 mA·h/g at 2 A/g). XRD shows that the introduction of MWCNTs can improve the reversibility via chemical interactions and accelerate the electron transfer in the Cu_1.85Se/MWCNTs. Notably, the assembled symmetric supercapacitor (SC) device expresses a high energy density of 41.4 W·h/kg, and the capacity remains 83% even after 8000 charge/discharge cycles. This research demonstrates the great potential of developing high specific capacity anode materials for superior performance supercapacitor.