Abstract: The relations of viscosity with pressure of phosphate synthetic oil and methyl silicone oil are studied at high pressure with an ultra-high pressure capillary viscometer. The solidification of phosphate synthetic oil at high pressure is preliminarily investigated by using infrared spectroscopy, nuclear magnetic resonance as well as the gel permeation chromatography. The slope of viscosity-pressure of synthetic phosphate oil is greater than that of methyl silicone oil. Before and after the experiment, the phosphate synthetic oil transforms from liquid into glass state and changes from transparent into milky white. However, no significant change in the structure of phosphate synthetic oil but increase in molecular weight can be found. Therefore, the phosphate synthetic oil experiences mechanical-chemical action under high pressure. After high pressure and high shear rate, the chemical reaction occurs. The high activity macro radicals generated by mechanical forces are responsible for the oxidation.