Heavily boron-doped microcrystalline diamond (BDD) films were prepared on single-crystal silicon substrates by hot-filament chemical vapor deposition, and then thermally oxidized in air at different temperatures and durations. The effects of thermal oxidation on the surface structure, wettability, electrochemical properties, and tetracycline degradation performance of BDD electrodes were systematically investigated. The results show that thermal oxidation induces a competition between surface activation and structural damage. Moderate oxidation can selectively remove non-diamond carbon from the surface and grain boundaries, expose more diamond facets, and introduce oxygen-containing functional groups, thereby improving surface hydrophilicity, reducing background current, and widening the electrochemical potential window. However, excessive oxidation at high temperature or for prolonged time causes surface etching, structural disorder, and degradation of the conductive network, resulting in weakened electrochemical performance. Among the investigated conditions, the BDD electrode treated at 700 ℃ for 30 min exhibits the best overall performance. Raman analysis indicates that this sample has the narrowest diamond peak and the highest intensity ratio of the diamond peak to the G band, suggesting an optimized balance between non-diamond carbon removal and preservation of the sp³ diamond framework. XPS results further confirm that thermal oxidation increases the surface oxygen content from 7.23 at.% to 17.45 at.%, mainly through the formation of C-O, C=O, and C-OH groups. Electrochemical measurements show that the optimized electrode has a wide potential window of 4.31 V and a low background current of 8.19 mA/cm². In tetracycline degradation tests, this electrode achieves 88.27% removal after 3 h and complete removal after 9 h. The chemical oxygen demand decreases to 52.0 mg/L, corresponding to a mineralization efficiency of 94.47%. These results demonstrate that appropriate thermal oxidation is an effective strategy for optimizing heavily boron-doped microcrystalline BDD electrodes for electrochemical water treatment.