Abstract: As an important load-bearing member of a double-arched tunnel, the law of stress variation for an intermediate rock pillar in the process of construction is complex. In this paper, based on a highway tunnel on provincial road 13, a three-axial stress test is carried out at the tunnel section K2+163. Testing results show that the three-axial stresses on different parts of the intermediate rock pillar are inconsistent with each other, in which the top and bottom parts of the rock pillar are loaded by vertical stress and the middle part is loaded by horizontal stress along the direction of excavation. As for the advanced excavation of a single tunnel, the tri-axial stresses inside the rock pillar approach their peak value when the working face is advanced to the monitoring section. On two vertical orthogonal planes parallel to the excavation direction and tunnel portal, an equivalent bending moment exists that is caused by stress action at the opposite direction. In addition, combined with engineering practices, the existing analysis formula is further developed to predict the tri-axial stresses of an intermediate rock pillar under of Class IV rock.