Single layer MoS2 is a two-dimensional direct band gap semiconductor material with a band gap of 1.8 eV, which can be used to develop new nano electronic and photoelectric functional devices. Because the vacancy in semiconductor can capture charge carrier and local exciton and form scattering center, which greatly affects the transport and optical properties of its main materials, the theoretical basis and numerical calculation algorithm of the first principle method are mainly used to study the influence of the original vacancy on the optical properties and saturated absorption of single layer MoS2. The presence of sulfur vacancy in the band gap results in a localized intermediate state, which leads to a significant red shift in the position of optical absorption peak and a strong absorption in visible light. Through the combination of figure and graphic, the absorption mechanism of the defect modulation can be understood well, which provides a deep guidance for the preparation of two-dimensional optoelectronic devices.