The structural stability, the electronic and the optical properties of Er-doped silicon nanoparticles are investigated by first principles based on the density functional theory. The results show that the structure is more stable when the doping concentration of Er atoms is smaller in silicon nanoparticles. The doping of Er atom in silicon nanoparticle introduces the impurity levels which result in the narrowing of band gap. A strong absorption peak occurs in the low-energy region of Er-doped silicon nanoparticles, and the intensity of the absorption peak decreases gradually, even disappears with doping concentration decreasing. The study provides the theoretical basis for the design of silicon-based materials.