By adopting the calculating analysis method of elastic-plastic three-dimension FDM numeric simulation，this paper makes a simulation analysis on the structure stability of bias-pressure construction stage of a double-arch tunnel passing through a? water-eroded groove. The research result shows that the mid-span of beams is a worst section of quadric lining and middle wall, instability and ripping damage easily happen here; the physics property of filling matter in water-eroded groove has distinct influence to the internal force state of structure; setting the invert in time is favorable to improve internal force state of supporting structure, and hence makes the structure more stability; the middle wall is a key part of tunnel structure, so some engineering measures ought to be adopted to improve its partial pressure state in construction process.