The role of the working medium used by the hydraulic press is not only to transmit pressure, but also to ensure that the working parts of the machine are sensitive, reliable, long-lived, and less leaky. The basic requirements for the working medium of the hydraulic press are: 1 It has suitable fluidity and low compressibility to improve the transmission efficiency; 2 It can prevent rust; 3 It has good lubricating performance; 4 It is easy to seal; 5 The performance is stable, long-term work Without deterioration. The hydraulic press initially uses water as a working medium and later uses an emulsion made by adding a small amount of emulsified oil to the water to increase lubricity and reduce rust. In the late 19th century, a hydraulic press using mineral oil as a working medium emerged. Oil has good lubricity, corrosion resistance and moderate viscosity, which helps to improve the performance of the hydraulic press. In the second half of the 20th century, a new type of water-based emulsion emerged. Its emulsified form was "oil-in-water" rather than the original "water-in-oil". The external phase of the "oil-in-water" emulsion is oil, its lubricity and corrosion resistance are close to that of the oil, and its oil content is small and it is not easy to burn. However, the price of water-based emulsions is expensive, limiting its promotion.