When an existing open station is converted into a terminal station arranged within the user’s premises, due to the generally compact area of the station, the reliability requirements for the grounding grid are higher, and the diagnosis and evaluation of the corrosion status of the grounding grid are becoming increasingly important. At present, the main method for diagnosing corrosion in grounding grids without excavation relies on electrical network theory analysis. However, this method is prone to having more branches than the number of observable nodes in diagnosis, leading to a high degree of uncertainty. Therefore, an improved seagull optimization algorithm is proposed, which integrates Gauss mapping and levy flight strategy on top of the traditional seagull algorithm, improving computational stability and convergence speed. A simulated grounding grid model is built, and the accuracy and reliability of the improved seagull optimization algorithm are verified through simulation calculations under various grounding grid corrosion conditions, combined with comparative analysis of other commonly used optimization algorithms. The simulation results show that the diagnostic deviation of the corrosion branch in the grounding grid using the improved algorithm is less than 5%, which is significantly lower than the other two traditional optimization algorithms, proving its ability to improve the accuracy of grounding grid corrosion diagnosis.