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In order to determine the degradation degree of in-use Water glycol fire-resistant hydraulic fluids (HFC) to grasp the oil change and replenishment timing, the mixed weight adjustment method and substitution synthesis method were applied to the comprehensive evaluation of HFC degradation degree. Based on the four design functions: anti-combustion, lubrication, anti-corrosion, and power transmission, the monitoring indicators were reduced, and an HFC monitoring indicators system including four indicators: moisture content, viscosity, pH, and contamination level was constructed. The trigonometric function, incremental exponential function and decreasing exponential function were respectively applied to the relative degradation degree assessment of moisture content and viscosity, contamination level, and PH. The subjective and objective weights of indicators were calculated by analytic hierarchy process and entropy method, and then combined and adjusted by the mixed weight adjustment method. The comprehensive relative degradation degree was synthesized by substitution synthesis method. The analysis example shows that the mixed weight adjustment method can achieve the "promoting good and punishing poor" function and the substitution synthesis method can meet the "one-vote veto" mechanism in oil monitoring. A more accurate comprehensive evaluation of HFC relative degradation degree could be made by mixed weight adjustment method and substitution synthesis method.