We report a first-principles theory for analyzing the parametric electron pump at finite frequency. The pump is controlled by two pumping parameters with phase difference Φ In the zero-frequency limit, our theory recovers the well-known adiabatic result that the pumped current I (Φ)～sinΦ. At a finite frequency, it predicts I (Φ=0)≠0 while I (Φ=π)≈0, consistent with recent experimental data. We discuss a possible mechanism behind the nonzero pumped current at f50 from photon-assisted processes.