As a significant parameter in tuning the structure and performance of the boron-doped diamond (BDD), the thickness was focused on the mediation of the boron doping level and electrochemical properties. BDD films with different thicknesses were deposited on silicon wafers by the hot filament chemical vapor deposition (HFCVD) method. The surface morphology and composition of the BDD films were characterized by SEM and Raman, respectively. It was found that an increase in the BDD film thickness resulted in larger grain size, a reduced grain boundary, and a higher boron doping level. The electrochemical performance of the electrode equipped with the BDD film was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in potassium ferricyanide. The results revealed that the thicker films exhibited a smaller peak potential difference, a lower charge transfer resistance, and a higher electron transfer rate. It was believed that the BDD film thickness-driven improvements of boron doping and electrochemical properties were mainly due to the columnar growth mode of CVD polycrystalline diamond film, which led to larger grain size and a lower grain boundary density with increasing film thickness.