The electron tunneling through a mesoscopic hybrid system, a normal-metal-quantum-dot-superconductor (N-QD-S) system where the intradot Coulomb interaction is neglected, in the presence of the time-varying external fields, has been investigated. By using the nonequilibrium Green-function method, the time-dependent current j L (t) and the average current ͗ j(t)͘ are derived. The photon-assisted Andreev tunneling ͑PAAT͒ and the normal photon-assisted tunneling ͑PAT͒ are studied in detail. In the

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... ase of បϽ⌬, where is the frequency of external fields and ⌬ is the energy gap of the superconducting lead, the average current ͗j͘ vs the gate voltage exhibits a series of equal-interval PAAT peaks, with negative peaks on the left-hand side and positive peaks on the right-hand side of the original resonant peak in the absence of the external fields. This is very different from the N-QD-N system. While for បϾ⌬, various PAT processes cause a rather complicated dependence of the current on the gate voltage. In addition, the current-bias-voltage characteristics become more complicated: each Andreev reflection peak is split into side-band peaks and each current plateau is split into substep plateaus.