Taking into account the case that the mass M and charge Q of Reissner-Nordstr?m black hole varying slowly with time t, the discussion on the quant um radiation characteristics of the black hole shows that the thermal radiation spectrum of charged Dirac particles is connected with the black hole's evaporati on rate together with the factors M(t), Q(t) etc. changing with time, and that the maximum energy of the quantum nonthermal radiation of nonstationary and slowly-changing Reissner-Nordstr?m black hole is equal to the chemical potential in the quantum thermal radiation spectrum.