Abstract: As the core structural component of double-arch tunnels, the force condition of the mid-pillar is not only related to the safety of the tunnel structure during construction but also directly affects the long-term safety and stability of the tunnel structure during operation. This paper takes the expansion of an existing single-tube tunnel into a double-arch tunnel as an example and carries out the model test and numerical simulation to study the stress characteristics of the mid-pillar during the construction. The construction plan is excavating the right tube (first tube) followed by the left tube (second tube). The results of the model test show that after completion of the expansion construction, the left side of the curved top surface of the mid-pillar is under tension while the right side is under compression; the left and right sides of the shoulder of the mid-pillar are both under tension, and the tensile stress on the right side is 2.86 times that on the left side; the left and right sides of the waist of the mid-pillar are both under compression, and the compressive stress on the right side is 1.32 times that on the left side, which are basically consistent with the numerical simulation results; the compressive stress at the toe of the left side of the mid-pillar is greater than that of the right side, and the mid-pillar is subject to bias pressure toward the left side of the tube. The horizontal displacement of the mid-pillar moves firstly to the side of the second tube then to the side of the first tube, and finally to the side of the second tube.