Using geothermal resources to instead of steam bled form the turbine steam to preheat feed water in a conventional Rankine cycle power station is also termed as geothermal aided power generation (GAPG), this offers a significant potential for the large-scale utilization of geothermal energy for power generation. GAPG enables the conversion of low temperature geothermal energy in the case of 90-260 degrees C into power at a higher efficiency. In GAPG system, geothermal heat is used to preheat the feed water of power generation, instead of the extraction steam, thus the steam flows through the lower stages turbine have changed. In other words, the lower stage turbine works under off-design conditions. However, previous simulation models for the GAPG and solar aided power generation (SAPG), have not considered this turbine off-design operation. Under the off-design conditions, the isentropic efficiency of the turbine and pressure, temperature of each stage of turbines change which lead to the change of the output of turbine. In this paper, a modified thermodynamic model has been developed, considering these changes, which then can estimate the performance of a GAPG plant more accurately than the previous models. The results from various models are compared in this paper.