In order to reduce the failure of pump station and hydraulic support filter caused by different hydraulic fluid contact reactions, it is necessary to predict the mixed stability of different hydraulic fluids. Four hydraulic fluid formulations that meet the standard requirements were selected, and the diluents were mixed according to the same mass ratio. The stability, conductivity, Zeta potential, surface tension and turbidity of the mixtures with different proportions at room temperature and 70 ℃ were investigated. The experimental results showed that the solutions of different mixtures were stable at room temperature after 168 h of standing, and floccules appeared in some mixtures at 70 ℃. The conductivity of the mixture was 5.04 - 14.03 mS/cm, Zeta potential was 13.63 - 67.49 mV, surface tension was 35.9 - 42.2 mN/m, and turbidity was 1.14 - 76 NTU; under the same proportion of mixing, high conductivity and low Zeta potential are not conducive to the thermal stability of the mixture; surface tension and turbidity are relatively weak in predicting stability.