Abstract: In order to eliminate foam harm to hydraulic system, improve defoaming performance of hydraulic fluid formulations, prevent insufficient hydraulic support motivation and ensure action accuracy of hydraulic support, defoamer matching of hydraulic fluid was studied. The nature of hydraulic fluid foaming, compatibility, anti-foaming, dose and persistence of defoamer was investigated. The intrinsic causes of compatibility and anti-foaming were analyzed. Experimental results showed that emulsifiers and lubricants were easy to trigger hydraulic fluid foaming, excellent properties of defoamer were showed when the dosage was slightly higher than the critical micelle concentration （ＣＭＣ）, the compatibility of defoamer and hydraulic fluid increased with the increase of the similarity for their chemical structure and different defoamer had different influence on surface tension of hydraulic fluid.