According to the inplane nonlinear elastic instability of circular arch with initial geometrical defects， the displacement loading device is designed to measure real-time loading-displacement curves in the loading process. And the actual deformations and unstable modes of circular arch are tracked

further

the buckling equilibrium path of circular is obtained

by which the up and lower instability limit points are captured. By comparing the measurement results with the FEA results

it can be revealed that how the span ratio and the defects of arch influence the instability limit points and unstable mode. It is showed that: 1) The initial geometric defects will influence the load-displacement curves of the arch

which makes the upper and lower limit stability points deviate from the ideal curves;2) Due to the initial geometric defect

the anti-symmetric deformation trend occurs in the unstable equilibrium path;3) With the increase of the ratio span

the instability limit load will improve

then the bearing capacity of the structure will enhance according;4) The effect of initial geometric defects on the pre buckling mode is more obvious than the post buckling mode.