The corrosion detection of main ground electrode in underground coal mine is crucial to the safety of the staff. Traditional manual visual inspection detection by transporting the electrode overground is time-consuming and the damage magnitude cannot be confirmed. Therefore, a new online corrosion detection method based on ultrasonic SH wave echo characteristics is proposed. Firstly, the dispersion equation of SH wave in the liquid-immersed plate structures is derived. The zero-order SH wave with small dispersion and long propagation distance is selected as the detection excitation signal, and the finite element model of liquid-immersed plate structure is constructed to determine the optimal ultrasonic excitation frequency. Secondly, the coupled flow-solid-acoustic multi-physical field detection model of the main grounding electrode in coal mines is constructed and the corrosion detection ability of ultrasonic SH0 wave on the main ground electrode is studied by simulation, then the echo signal characteristics under 1~5mm corrosion depth and 5~25mm corrosion radius are analyzed. Finally, the non-destructive testing system of the main ground electrode in coal mine is constructed, and experimental verification is carried out. The experimental results prove the feasibility of the proposed method, the corrosion location error is 5.64% on a 500mm×375mm×5mm main grounding plate, the corrosion signal amplitude is positively correlated with the magnitude of corrosion defects. The research provides an effective method for corrosion location and damage assessment of underground coal mine main ground electrode.