High humidity in the welding rod can lead to obvious quality problems such as bubbles and porosity in the weld. In order to meet the non-destructive detection of welding rod humidity in the field, a flexible interdigital capacitive rod humidity detection method is proposed. Firstly, a finite element model was established based on the new flexible interdigital capacitive sensor to study the influence of the structural parameters of the interdigital electrode on the sensor performance. Then, the parameters of the structure of the interdigital capacitive welding rod humidity sensor were optimized using BP neural network and genetic algorithm, and the optimal design of the humidity sensor was obtained. Finally, the U-shaped elastic mechanism was designed to completely wrap the welding rod and complete the humidity testing and calibration experiments for J507 and J607 welding rods. The simulation and experimental results show that the sensor detection depth is about 1. 4 mm (less than the thickness of the electrode flux) when the structural parameters of the interdigital electrode are optimal, and the goodness of fit of the electrode humidity value to the capacitance value reaches 0. 98. This method has good detection capability for welding rods with humidity values in the range of 0. 117% to 2. 5%, which provides a new idea for realizing on-site, nondestructive detection technology of welding rod humidity and helps to avoid welding defects during the welding process.