Synthetic study on CO2-saturation and pressure-buildup differentiation by using VSP-FWI is conducted in a virtual CCS site. Geophysical and Reservoir simulation works are first implemented to create geophysical models before and after CO2 injection. Virtual VSP-FWI monitoring experiment is then implemented on the models. In parallel with this, simple linear equations to estimate CO2-saturation and pressure-buildup are derived. Inverted P-and S-wave velocities through the VSP-FWI are found to be highly accurate, and used in the derived equation. As a result the saturation change and reduction of differential pressure are estimated consistently with the values after reservoir simulation. This result indicates that CO2-saturation and pressurebuildup can be differentiated around the well, if both P-and S-wave velocities are accurately inferred by applying VSP-FWI and relevant rock-physics model is known.