Based on the free electron gas model, the plasmon frequency of a monolayer-atom system is solved by using the linear response theory and the Green function method, and the analytic expression for the plasmon dispersion, which can be applied to high electron density and short wavelength cases, is obtained. The results show that the thickness of the atomic layer can reduce the plasmon frequency. The thicker the atom layer is, the smaller the plasmon frequency will be, and this reduction is more obvious when the wave vector is larger. When the thickness of the monolayer-atom system tends to zero, the plasmon dispersion of the monolayer-atom system tends to that of a pure two dimensional system. In addition, in the first-order approximation of the plasmonfrequency in monolayer-atom system, it is found that the relative correction of the plasmon frequency in the case of long-wave approximation is linearly related to the wave vector and the thickness of the atomic layer.