Excitons and phonons are the two fundamental types of quasiparticles in solids including atomically layered two-dimensional (2D) transition metal dichalcogenides (TMDs). They themselves and their interactions play a central role in determining the optical properties of 2D-TMDs. Herein, we present a combined study on effects of environmental dielectric screening on the optical properties of excitons and the exciton-phonon scattering in a series of WS2 monolayers with different environmental dielectric screenings via reflectance spectroscopic measurements and microscopic calculations. As the dielectric screening increases, the linewidth of the A-exciton resonance structure exhibits a significant narrowing and less dependence on temperature. Microscopic theoretical analysis based on the variational Ansatz shows that the noticeable reduction in both radiative recombination rate and phonon-assisted intervalley scattering of excitons result in the observed phenomenon due to the reduction of exciton Bohr radius caused by the increase of the dielectric screening. In this study, we demonstrate that dielectric engineering can offer an effective approach for regulating the excitonic effects and investigating the complicated exciton-phonon interactions in ultimate 2D monolayer systems.