Model test is designed in order to study the variation of force of existing tunnel and the relationship between force and load-bearing arch under excavation and filling. The variation law of the tunnel stress is obtained by investigating the displacement, internal force and surrounding rock pressure of the tunnel during excavation and filling. And the relationship between tunnel force and load-bearing arch is analyzed. The results show that: (1) Due to the existence of load-bearing arches, the excavation process above the existing tunnel will go through three stages of no influence, weak influence and strong influence; but there will be no load-bearing arch during the filling process above the existing tunnel, and the influence of filling on the tunnel will basically increase linearly, the tunnel continues to be affected. (2) The structural stress after filling is generally greater than that of excavation in the same covering height, and the crown and invert are affected more obviously than other parts, which are liable to crack under internal tension. (3) Whether the load-bearing arch is generated or not mainly depends on the stress state of surrounding rock of the tunnel at a certain buried depth undergoes a process of three-stress to two-stress transformation; the main reason of loadbearing arch formation is the stress transfer and redistribution of surrounding rock during tunnel excavation.