We derive a novel time-domain asymptotic solution for a transient scattered field by a cylindrically curved conducting surface by extending the corresponding frequency-domain asymptotic solution. The transient scattered field is excited by the high-frequency modulated pulse source. By comparing the asymptotic solution with the reference solution calculated numerically, we show that the novel time-domain asymptotic solution is highly accurate and very efficient in terms of the computation time. We also show that the time-domain asymptotic solution is easy to understand the physical insight of the scattering by the cylindrically curved surface.