Abstract: An in-situ robot material-reducing processing method and system for a hydraulic turbine top cover are provided. The method includes: S1, obtaining frequency response data of an end of a cutter controlled by a robot by performing an impact hammer test; S2, obtaining a modal parameter of the end of the cutter controlled by the robot by using a modal analysis software; S3, obtaining a damping matrix [C] and a stiffness matrix [K] by using a free vibration equation of a damped system; S4, establishing a dynamic model of a three-degree-of-freedom robot processing system; S5, obtaining a milling force coefficient of the cutter by performing a calibration experiment; S6, solving a dynamic equation; S7, drawing a lobe diagram of flutter stability of a milling process performed by the robot; and S8, obtaining stable milling process parameters according to the lobe diagram of flutter stability.