The pressure-viscosity relationships of phosphate synthetic oil and other two kinds of similar atmospheric viscosity synthetic oils were studied using ultra-high pressure capillary viscometer. The pressure-viscosity relationship of phosphate synthetic oil is much better than the other two kinds of synthetic oils. The impact of pressure on viscosity is not limited to purely physical factors. The pressure can cause a variety of chemical reactions in some cases. The investigation results of mechanochemistry of phosphate synthetic oil at high pressure reveal that the physical state of phosphate synthetic oil changed from liquid into glassy amorphous state under high pressure, and the color varied from transparent into milky white. The mechanochemistry of phosphate synthetic oil was analyzed using the infrared spectroscopy and gel permeation chromatography, and the results indicate that under high pressure, the oxidation reaction of phosphate synthetic oil occurred, and the molecular weight distribution changed with the increase of the low molecular weight region. The reason of the mechanochemistry phenomena was that phosphate synthetic oil molecular chain disconnects to inform great radical. The great radical has strong activity, and reacts with other free radicals acceptor (oxygen, etc).