The half-metallic (HM), magnetic, and optical properties of the (001) surface of Ge doped half-Heusler alloys Mn2GexAs1-x (x= 0.00, 0.25, 0.50, 0.75, and 1.00) are predicted by first-principles method. Among the ten different kinds of Mn2GexAs1-x (x= 0.00, 0.25, 0.50, 0.75, and 1.00) (001) surfaces, the HM character is only observed in the MnMn-term of Mn2Ge (001) surface which represents the priority for optical property in the red visible light region, and the MnAs-term of Mn2As (001) surface represents a very high spin polarization P of -97.5% using generalized gradient approximation (GGA) method (-98.4% using GGA+ U method), which represents the priority for optical property in the blue visible light region. The spin polarizations of the rest eight kinds of surfaces are very low, thus these surfaces can't be used in spintronic devices. In the energy range of 3.1-15.0 eV for the ultraviolet light, the light absorption capacity of the HM MnMn-term of the Heusler alloy Mn2Ge (001) surface is better than that of the nearly HM MnAs-term of the Heusler alloy Mn2As (001) surface.