We present a numerical approach for solving time-dependent quantum transport problems in molecular electronics. By directly solving Green's functions in the time domain, this approach does not rely on the wide-band limit approximation thereby is capable of taking into account the detailed electronic structures of the device leads which is important for molecular electronics. Using this approach we investigate two typical situations: current driven by a bias voltage pulse and by a periodic field, illustrating that the computational requirement is no more than an inversion of a relatively small triangular matrix plus several matrix multiplications. We then present numerical results of time-dependent charge current for a one-dimensional atomic chain. The numerical solution recovers known results in the wide-band limit, and reveals physical behavior for leads with finite bandwidth.