The thickness of overburden of extra-long tunnels often amounts to hundreds of meters，even goes beyond 1000 m. Here， an extra~long tunnel whose thickness exceeds 500 mis called extra~U)ng tunnel with superthick overburden. Their depth in aquifers is big and the initial discharge mainly comes from elastic yield. Moreover，the permeability of wall rock gets ofter poorer and poorer with depth. In view of the above aspects， the tunnels are formularized as the wells near boundaries， and the calculation model for maximum discharge rate is got by way of method of images. Having been simplified properly， the model tallies basically with part of the existing relative models.