Jet-pipe servo valve will produce howling phenomenon in the process of installation and adjustment, resulting in high frequency vibration of the servo-valve spring tube, so that the spring tube quickly fatigue fracture, causing catastrophic accident. In order to solve this problem, Fluent was used to analyze the flow field of the pre-stage of the jet-pipe servo-valve, and the mechanism of jet-pipe servo-valve howling was analyzed; particle imaging velocimetry (PIV) technology was used to verify the visualization of the pre-stage of the jet-pipe servo-valve, and the visual images of the internal flow field and the vortex distribution characteristics of the pre-stage of the jet-pipe servo-valve were obtained and the howling phenomenon was eliminated through structural improvement. The results provide reference for solving the howling problem of jet-pipe servo-valve and optimizing the jet-pipe pre-stage structure.