  We have studied the evolution of the massless scalar field propagating in
time-dependent charged Vaidya black hole background. A generalized tortoise
coordinate transformation were used to study the evolution of the massless
scalar field. It is shown that, for the slowest damped quasinormal modes, the
approximate formulae in stationary Reissner-Nordstr\"{o}m black hole turn out
to be a reasonable prescription, showing that results from quasinormal mode
analysis are rather robust.
