The essence of shaft friction of jacked pile is friction at the pile-soil interface; but the mature tribology theory has not been used to study the mechanism of shaft friction yet. In this paper, friction mechanism of adhesion and deformation in tribology is introduced systematically. Using it to explain the sliding friction mechanism at the pile-soil interface of jacked pile, a good results is obtained. It provides a new approach to research the shaft friction of pile. The study suggests that unlubricated friction between jacked pile and soil is external friction, the sliding friction between jacked pile and soil is lubricated friction in mud that causes the shaft friction in the jacking process far less than that in load test. A pause during pile jacking can give rise to a sharp increase of pile jacking pressure. The principal cause for the increase of pressure is that the disappearance of the lubricating film of mud that brings the friction between jacked pile and soil from lubricated friction into unlubricated friction; the friction between jacked pile and soil in load test is unlubricated friction before failure; the strengthening of shaft friction near the tip of pile is due to the increase of tangential force caused by the improvement of soil strength underneath the pile that improves the coefficient of friction between piles and soil.