To evaluate the quality and damage condition of the electrolyzer, wave velocity detection technology and impact echo technology were used to detect the cathode part of the electrolyzer. The experimental results show that wave velocity is linearly related to the porosity, and there is also a linear relationship between wave velocity and the square root of reciprocal density in cathode carbon blocks (CCBs) before installation into electrolyzer. Combined with detection results of wave velocity and voltage drop, the large-size CCBs with relatively good quality can be found. Through the impact echo technology on cathode steel rods (CSRs), the results of the on-site detection show that the damage condition of CSRs can be effectively evaluated, and the damage location of CSRs can be determined. This study proposes a novel and quantifiable method for the evaluation of cathode quality and damage, which provides a reference for prolonging the service life of the electrolyzer.