Thermal effect is one of the important factors affecting the safe operation of high-power fiber laser system. Exploring the source of thermal effect of optical fiber laser system, actively carrying out research on heat-control technology, taking reasonable measures to suppress heat concentration, and greatly improving the mode instability threshold of optical fiber laser system to avoid mode degradation, are of great practical significance to further improve the safe and stable output power of optical fiber laser system. Taking widely-used concentrated end-pumping technology as an example, this paper summarizes the main sources of thermal effect of high-power continuous-wave fiber laser system, and puts forward practical solutions and reasonable suggestions for different thermal effects. Finally, this paper focuses on long-distance distributed side-pumping technology and pump-gain integrated functional fiber, and looks forward to the future development of 10 kW-level ultra-high power fiber laser.