BDD (Boron-doped Diamond) electrode may hold a promising application to detect heavy metal ions for actual water monitoring and early warning, but a poor understanding of influence mechanism of B/C ratio on detection performance is in the way of its fabrication and application. This work is intended to reveal the double-side effect of B/C ratio on detection performance of BDD electrode so as to facilitate its actual application. SBDD (Self-supported Boron-doped Diamond) electrode is introduced for the first time to get rid of the interference factors such as substrate. A systematic investigation is conducted for the influence of B/C ratio on microstructure and electrochemical behavior of SBDD electrodes. With the increase of B/C ratio, the grain size continuously increases, and the preferred orientation gradually changes from plane (220) to (111). The gradual increasing of impurity phase content indicates a deterioration of diamond phase quality. In addition, the electrode electrochemical behavior initially gets better then worse. SBDD electrode with a B/C ratio of 1/500 has the largest active surface area of 2.1 cm2, the smallest diffusion resistance and the highest signal response. Under optimal parameter set, the SBDD electrode enjoys a sensitivity of 0.42 μA L μg−1 cm−2 and a detection limit of 1.12 μg L−1 in a wide linear range of 5–120 ppb. The phase quality and grain morphology jointly contribute to the double-side effect. A suitable B-sp3-C content, preferred orientation of (111) and small particle size may make the performance improvement of BDD electrode available.