CuO is a promising but unstable photocathode in solar water splitting. Herein, a flexible CuO photocathode is prepared and its degradation mechanisms and stabilization strategies have been discussed. Briefly, we find alkali environment and low light intensity are the critical factors in the stabilization of the CuO photocathode. For practical usage, a composite semiconductor layer, composed of TiO₂, La₂O₃ and NiO, is deposited on the CuO photocathode, which is proved to be effective for enhancing the stabilization of the CuO photocathode. 100% of the photocurrent density has been retained after 20 minutes of continuous illumination. The optimized stable photocurrent density is measured as 0.3 mA cm⁻² at 0.5 V_RHE.