The horizontal axis tidal turbine (HATT) is a device that harnesses the energy of ocean currents and converts it into electrical energy. The blade plays a crucial role in the efficiency of power generation in HATTs. This study focuses on the use of slotted blades to enhance the efficiency of HATTs and investigates the flow control mechanism of these slots using computational fluid dynamics (CFD) methods. Initially, CFD simulations were conducted to analyze the impact of the slot’s geometry parameters on a two-dimensional (2-D) slot and to demonstrate its passive fluid control mechanism. Subsequently, the slot was implemented on three-dimensional (3-D) blades to examine its effect on the hydrodynamic performance of the blades. The results of the 2-D simulation indicate that the width and position of the slots have a significant influence on the lift-to-drag ratio of the hydrofoils, resulting in a maximum increase of 166%. For the 3-D blades, the simulation results reveal that the slot can enhance the power coefficient of the blades, particularly at low tip-speed ratios, with a maximum increase of 7.8%.