Oceans and rivers contain abundant renewable low-velocity flow energy without large-scale utilization. Here, we present a novel energy harvester based on vortex resonance to collect the low-velocity flow energy. The harvester is mainly constituted by a cantilever beam, a hollow cylinder, a carrier sheet, and a Solid-Liquid Triboelectric Nanogenerator (SLTENG). Computational fluid dynamic (CFD) and finite element analysis are used to simulate the vortex shedding frequency and the natural frequency of the harvester. After optimization of the structure, the natural frequency is close to the vortex shedding frequency (2 Hz) and reaches resonance. Furthermore, the state of the liquid inside the cylinder is analyzed by CFD under different volumes and diameters of the cylinder.