Nowadays, there is an increasing demand for wearable, portable and foldable small electronic products, and human-computer interaction interface devices. Therefore, the supercapacitors as the energy storage device were extensively studied owing to their high energy/power density, fast charge-discharge processes, and long cycle life. Wherein the flexible electrode material is the essential component to boost the performance of supercapacitors. Cellulose, as a kind of natural flexible material with low-cost, wide-sourced, renewable, and robust mechanical properties, have been used as flexible substrate or template of electrodes. To enhance conductivity and excellent electrochemical performance of the cellulose-based flexible electrode, the carbon nanotube (CNT) with high-conductivity, good thermal and chemical stability, and unique internal structure was integrated. Thereby, the cellulose/CNT-based flexible electrodes with high energy/power density and long cycle life performance of flexible supercapacitors are prepared. This review mainly focuses on cellulose/CNT, emphatically summarizing the composition, preparation, and mechanism of the cellulose/CNT-based composite flexible electrode for supercapacitors. Additionally, the current challenges and prospects of the cellulose/CNT-based composite flexible electrode are discussed.