To adapt to the low-velocity water flow closely related to human life, the natural energy can be efficiently harvested and used to power monitoring devices. Herein, a triboelectric soft fishtail (TE-SFT) driven by flow-induced vibration (FIV) effect is proposed based on the soft material synthesis technology. Specifically, inspired by the fishtail fin, a bluff body with the cross-section of fishtail-shaped is designed, and has a preferable vortex effect by fluid simulation. In power generation part, the triboelectric nanogenerator (TENG) is designed to act as an inertial pendulum structure by geometric method. Under the FIV effect, the TE-SFT driven by fishtail-shaped bluff body swings like a fish in the water and then brings the inertial pendulum to acquire the oscillation for harvesting energy from low-velocity water flow. The TE-SFT attains an open-circuit voltage (V_OC) of 200 V to 313 V at the flow velocities of 0.24 to 0.89 m/s. Additionally, after 30 days of water immersion, the V_OC of TE-SFT retains 96.81%. In demonstration, the TE-SFT is applied to power the temperature and humidity sensor through harvesting water flow energy. This work also provides a way for self-powered system based on the TENG and soft bionic fish in water environment.