With the unique physical and chemical properties, WS₂ shows great potential for applications in the fields of electronics and optics. Based on the first-principles calculations of density functional theory, the aim of this paper is to investigate the electronic structure and optical properties of single 4d transition metal atoms X (X=Nb, Mo, Tc, Ru, Rh, Pd) substitutionally doped monolayer WS₂. The results show that the transition metal atoms doped WS₂ systems are all exothermic and stable, and the decrease of the band gap width leads to the enhancement of conductivity and significant changes in electronic structure. For these doped metal atoms, Nb doped WS₂ exhibits metallicness, Ru doped WS₂ exhibits semi-metallicness, and Tc, Rh and Pd doped WS₂ induce magnetism. The dielectric constant and optical refractive index of Nb, Ru, Rh and Pd doped WS₂ systems increase. The WS₂ systems before and after doped have good transparency characteristics, and the absorption spectrum is red shifted. The absorption of Nb, Ru and Rh doped WS₂ is enhanced in the infrared region, and the absorption of Nb, Rh and Pd is enhanced in the visible region. Especially, the Pd doped WS₂ has the best absorption effect in the visible region, which has a certain potential for application in the photodetector.