We present herein the synthesis of single-crystal Co0.85Se nanobelt through a triethylenediamine-assisted solvothermal reaction. When it is applied as the counter electrode for quasi-solid-state dye sensitized solar cell, the single-crystal Co0.85Se nanobelt counter electrode exhibits excellent electrocatalytic performance for the reduction of I-3(-), which is attributed to the single-crystal nature and unique two-dimensional structure. As compared to the polycrystalline Co0.85Se, the single-crystal Co0.85Se has higher conductivity, thus exposing more active sites for electrocatalysis. The quasi-solid-state dye-sensitized solar cell with the single-crystal Co0.85Se nanobelt cathode achieves a power conversion efficiency of 9.06%, which shows significant advantages over polycrystalline Co0.85Se counter electrode that yields efficiency of 7.87% and even surpasses the Pt control (7.67%). More importantly, the quasi-solid-state dye-sensitized solar cell based on the single-crystal Co0.85Se nanobelt counter electrode presents excellent long-term stability under one-sun soaking.