Most of general geothermal simulators assume the pure water as a geothermal reservoir fluid. However, since it actually contains chemical species, the geothermal reservoir performances predicted by these general simulators may be different from actual ones. Therefore, in this study, we tried to develop the geothermal reservoir simulator that can take the effects of chemical species in water phase and chemical reactions between fluid and reservoir rock into consideration. The simulator developed in this study can deal with not only the water/steam flow and associated temperature change but also the chemical reactions such as dissolution of rock minerals and precipitation of solutes. That is, the chemical species concentrations are simulated in accordance with the equations expressing the kinetic reactions, convection and dispersion of chemical species. The effects of chemical species concentration such as elevation of boiling point, heat of reaction and changes in porosity/permeability are also considered. To verify the performances of this simulator, the results predicted by this simulator assuming no chemical reactions are compared with those by the commercial thermal simulator, followed by the validation of the accurate calculation for the influences of chemical species with analytical solutions. Finally, we conducted the case studies to examine how the chemical composition and/or temperature of recharge water affected geothermal reservoir behavior. Through these studies it was revealed that (1) the injection of the water with low salt concentration induced the dissolution of rock minerals while that of the water with excessive ion concentration resulted in precipitation of solutes near the injection well, (2) the change in reservoir temperature caused the mineral dissolution/precipitation due to the alteration of chemical equilibrium and (3) the magnitude of porosity and permeability changes associated with dissolution/precipitation of minerals might not be large.