We calculate the energy-loss function for monolayer phosphorene in the framework of time-dependent density functional theory. The calculations are performed in the adiabatic local density approximation with local field effects. We study the origin of the features in the absorption spectra and the energy dispersion of the features in the excitation spectra. The energy dispersions show a strong directional dependence. At vanishing momentum transfer, the excitation spectra are dominated by a plasmon peak at 10.5 eV. At finite momentum transfer, the plasmon dispersion can be described by the layer electron gas model.