Disclosed are a structure of an intermediate wall of a three arch excavated tunnel, which effectively exhausts subterranean water from a peripheral ground, and a method for constructing the tunnel. The method comprising the steps of: (a) excavating an upper portion of a central tunnel 10; (b) excavating a lower portion of the central tunnel 10; (c) forming an intermediate wall 20 and grouting a gap formed in the intermediate wall 20; (d) excavating an upper portion of a left main tunnel 30; (e) excavating an upper portion of a right main tunnel 40; (f) excavating a lower portion of the left main tunnel 30; (g) excavating a lower portion of the right main tunnel 40; and (h) installing a drain board 50 and a waterproof layer 60 and casting lining concrete 70 therein so that the tunnel is easily drained through drain pipes 65 and residual water pressure is eliminated.