A regulating valve cavitation diagnosis method and a testing system thereof are used for evaluating and diagnosing whether cavitation of a regulating valve occurs in an operation process. The regulating valve cavitation diagnosis method of the invention comprises the steps of calculating a flux coefficient Kv and a fluid Reynolds number Re in current aperture, and drawing an Re-Kv relation curve chart, and determining whether cavitation occurs through the Re-Kv relation curve chart. Furthermore the invention provides a testing system applied to the method. The testing system comprises a regulating valve testing pipeline system, a signal measuring device, a signal acquiring and transmitting device, and a cavitation diagnosis software system. According to the regulating valve cavitation diagnosis method and the testing system thereof, through a fluid Reynolds number-regulating valve flux coefficient relation curve chart of the regulating valve at a certain aperture, wherein the curve chart is drawn through the cavitation diagnosis software, a cavitation operation condition of the regulating valve in the current aperture can be acquired in a relatively accurate manner, and cavitationevaluation diagnosis of the regulating valve is realized.