In order to convert more vortex-induced vibration energy into electricity, a piezoelectric energy harvesting system using a modified circular cylinder is studied to evaluate its performance in water. The circular cylinder is often used to induce the Karman vortex street. To produce stronger and more regular vortices, a circular cylinder with a slit and concave rear surface is adopted in this paper. The systems using the two kinds of vortex shedders are simulated with physical fields fully coupled, respectively. The lift forces on the cantilever beams are computed and compared, as well as the output voltages of the piezoelectric film. The corresponding experiment is conducted to verify the simulation result. Both of them prove the effectiveness of the modified energy harvesting system in improving the harvested and output energy.