Effective fluid bulk modulus is an important parameter in hydraulic systems. The major factors of effective fluid bulk modulus were analyzed, and a mathematical model with entrapped air content and working pressure at constant temperature was developed. A device for removing air content based on the method of vacuum-pumping in a closed reservoir was developed to increase bulk modulus. And bulk modulus was measured by the device when vacuum-pumping had been completed. Experiments were carried out in a large hydraulic system, and the results showed that vacuum-pumping is an effective method to reduce air content, and the device increased fluid bulk modulus from 821.3MPa to 1201MPa. Bulk modulus can be improved by using this method in the closed hydraulic system.