Development of high-quality porous BDD film is of significant to materials science and technology due to its intriguing physical and chemical properties, however, inescapable emergence of non-diamond carbon (NDC) during the deposition of porous BDD makes it an arduous challenge. In this contribution, we proposed a new template-free way to controllably fabricate a series of 3D porous BDD films through chemical vapor deposition (CVD) of different diamond/NDC ratio of composite films and complete remove of NDC. The porous BDD films shown the excellent structure properties, such as NDC-free diamond, 3D interconnected porous microstructure and tunable pore size, as well as boosting electrochemical properties (high oxygen evolution potential (>1.8V), accelerated electron transfer rate and 390-870 folds improvement of capacitive current). The marriage of the superior microstructure and unprecedented electrochemical properties makes the porous BDD films promising electrodes for electrochemical applications in double-layer capacitor. The large capacitance of 17.54 mF/cm² in diamond electrodes and 100% of capacitive retention were obtained, this resulted from the significant enhanced effective electroactive surface area, speed-up diffusion of electrolyte and excellent structural stability of the porous BDD electrode. Overall, the new method bridges the gap in simultaneous acquiring high-quality diamond and large surface area for the fabrication of BDD film and proposed a promising candidate in the various electrochemical applications.