Abstract: The objective of this research is to employ electrical resistivity technology to address key issues in underwater riprap dams, such as defect detection and quality evaluation. The study investigates the electrical resistivity characteristics of underwater riprap under varying supply voltage and void fraction conditions using the four-electrode method. An empirical formula is established to correlate electrical resistivity with the occurrence state of underwater riprap. The research reveals that the electrical resistivity of underwater riprap rapidly decreases with increasing supply voltage (10-60 V), stabilizing at higher voltages (>30 V). The electrical resistivity also decreases with increasing void fraction until it stabilizes. The relationship between electrical resistivity and void fraction shows similar patterns for different supply voltages. The power, exponential, and logarithmic functions are used to fit the relationship between electrical resistivity and void fraction, with the power function providing the smallest error. The empirical formula for void fraction and electrical resistivity of underwater riprap is then established, taking into account the influence of supply voltage. The empirical formula's reliability is verified through an underwater riprap dam model test, enabling effective description of the occurrence state of underwater riprap and quick identification of internal defects in underwater riprap dams.