A theoretical study was carried out to analyze the impact of composite effect on dielectric constant and tunability in the ferroelectric-dielectric system. Models of Ba0.5Sr0.5TiO3-Mg2TiO4, Ba0.45Sr0.55TiO3-MgO and Ba0.5Sr0.5TiO3-MgO-Mg2SiO4 system were constructed. The corresponding dielectric constant, tunability, and electric field distribution were obtained from the finite element analysis and the connections between these parameters were analyzed. The effects of the relative relationship between the dielectric constant of ferroelectric and dielectric on the whole dielectric properties were also analyzed by constructing a series of models with different dielectrics.