The emerging two-dimensional (2D) materials exhibit strong exciton effects with rich exciton types, dominating their optical and optoelectronic properties. The modulation of the interlayer van der Waals (vdW) gap in 2D materials significantly influences interlayer coupling, enabling the manipulation of the excitonic binding energy (E_B), electronic band structure, etc. However, the impact of the vdW gap between 2D materials and their substrates on excitonic behavior is seldom explored, and the physical mechanism remains unclear. Here, we experimentally demonstrate the vdW gap between 2D materials and substrates can effectively tune the excitonic E_B and electronic bandgap, owing to the change in the local dielectric environment and the Coulomb screening effect. The vdW gap between monolayer WS₂ and SiO₂/Si substrate reduced from ~ 6 to 3 nm by the simple annealing process enhances the Coulomb screening effect, decreases the excitonic E_B by ~ 20 meV, redshifts the bandgap by ~ 14 meV and thus strongly suppresses the trion formation. Our findings elucidate the underlying physical interaction mechanisms between 2D materials and substrates, offering valuable insights for designing and optimizing optical and optoelectronic devices by utilizing these supported 2D materials.