Sediments detection on the grading electrodes installed in the inner cooling system is the necessary part to ensure safe operation of the high voltage direct current (HVDC) converter valve. At present, the sediments detection of the grading electrodes mainly depends on manual screening, which has great blindness and is easy to cause system leakage and other faults. To address these issues, an online sediments detection method based on the echoing characteristics of ultrasonic guided wave is proposed. The L(0,2) mode guided wave with low dispersion and long transmission distance is selected as the excitation signal. Sediments of electrodes are considered as porous medium and the characteristic parameters are calculated. Then, a fluid-solid-acoustic multi-physical field simulation model of sediments detection is formulated, and the optimum excitation frequency is determined. The reflection and absorption characteristics of 0. 2 ~ 1. 0 mm thick sediments on acoustic signals are simulated. The mode conversion processes in the interaction between the longitudinal L(0,2) mode and sediments are analyzed. Finally, the experimental system is established to verify the actual detection accuracy in the sediments thickness of 0. 1~ 0. 8 mm. Experimental results show that the accuracy of online sediments detection method based on the echoing characteristics of ultrasonic guided wave is up to 0. 1 mm, which demonstrates the online sediments detection method is effective for sediments detection with grading electrodes of the HVDC converter valve cooling system.