The electronic structures, half-metallic and optical properties, as well as formation energy of pure LiZnAs, Mn-doped LiZnAs and Mn-doped LiZnAs with excess and deficient of Li are geometrically optimized and calculated by using the first principle density functional theory based on the full potential linearized augumented plane wave method. Results show that in the systems of Li(Zn0.875Mn0.125)As, Li1.1(Zn0.875Mn0.125) As and Li0.9(Zn0.875Mn0.125)As a 100% spin injectors is revealed, and the materials exhibit half metallic. The half metallic materials with excess and deficient of Li are more stable than Mn-doped LiZnAs. Excess of Li could improve the Curie temperature and conductivity of the material, and cause the formation energy of the system decrease. So the separation of spin and charge injection mechanisms may be achieved in LiZnAs semiconductor, and the magnetic and electrical properties of diluted magnetic semiconductor may be regulated respectively by Mn doping and Li stoichiometry. In addition, the dielectric function and the complex refractive index function in the low-energy region are found to be influenced by the stoichiometry of Li.