A one-dimensional mathematical model of a steam-water two-phase injector is presented. This model offers a method of estimating critical conditions of steam at the site of the motive nozzle throat, based on the local sound velocity in that area. Fluid thermal properties were based on a real fluid approach, where the CoolProp database was utilized. A different method was adopted to formulate governing equations for all passages of the injector based on the principles of the conservation of mass, momentum, and energy. The pressure profiles of the injector at different inlet steam pressure and inlet water pressure were used to validate the proposed model; they agreed well, with a maximum relative rate of error within 9.5%. Based on the validated model, the influence of the different area ratios and coefficients of the diverse sections on the performance of an injector used in district heating was investigated. The main inlet parameters - steam pressure and water pressure - were within the range of 0.20-0.60 MPa and 0.14-0.49 MPa. The exergy destruction rate for every steam-water injector component was also computed. The results illustrated that the injector discharge pressure increases with the throat area ratio of the motive nozzle and mixing chamber. The isentropic efficiency coefficients of the converging section and diverging section of motive nozzle affects the entrainment ratio and compression ratio differently. The main irreversibility occurs in the steam nozzle (41.34%) and mixing chamber (57.95%). The exergy efficiency of the injector decreases with the increase of the mass entrainment ratio. It also increases in coordination with the increase of inlet steam pressure, and decreases with the increase of inlet water pressure.