The present disclosure relates to a gain optical fiber heat-dissipating device for high power ultra-fast laser, including a gain optical fiber and a heat-dissipating structure. The heat-dissipating structure includes a metal tube, a flexible heat-conducting layer and a water-cooling structure. The gain optical fiber is passed through the metal tube, and the flexible heat-conducting layer is provided between the metal tube and the gain optical fiber. The water-cooling structure is provided on the metal tube to reduce temperature of the gain optical fiber. The gain optical fiber heat-dissipating device according to the present disclosure can dissipate the heat through a water-cooling mode, and realize rapid heat dissipation, thus improving heat-dissipating efficiency.