The influence of track skeleton for track support of TBM tunneling on drainage capacity of a diversion power tunnel is estimated by analytical method and numerical method. The drainage capacity needed, which can guarantee the safe tunnel construction under different water gushing volumes, can be predicted by putting the comprehensive roughness gained from the numerical method into the analytical formula. The abovementioned method has been applied to practice. The results show that the calculated drainage capacity coincides with actual one well so as to prove the accuracy and practicability of the method.