The invention discloses a method and a system for detecting a fault of a rotating diode of a brushless exciter. The method includes steps of: obtaining, from a preset shooting position, a fuse blown image including a pop-up end of a fault state indicator, and identifying a reference graph and a state graph from the obtained fuse blown image; respectively calculating the center position of the reference graph and the center position of the state graph by using a preset algorithm; and determining whether a fuse is blown by comparing whether the distance between the center of the state graph andthe center of the reference graph exceeds a preset threshold. The method for detecting the fault of the rotating diode of the brushless exciter based on image recognition analyzes and monitors the fuse image of the fuse and can effectively determine whether the rotating diode is faulty, the judgment result is accurate and reliable, and the problem that the stroboscope observation method cannot achieve online detection is effectively solved. The method and the system are of great significance for ensuring the safe and stable operation of the brushless excitation generator set.