The chimney junction of Solar Chimney Power Plant (SCPP) was the main element coupled the solar collector to the chimney. It presents an important impact on the SCPP design, which is change the airflow direction. The numerical code "ANSYS Fluent" was used in this study to investigate how the chimney Junction radius affected the local airflow characteristics and the turbine site. Using test results from an experimental prototype constructed in Sfax, Tunisia, the numerical method was validated and verified. For four setups with various chimney connection radii, thermodynamic variables such as distributions of the total pressure, static temperature, static pressure, and magnitude velocity were examined. Also, the influence of these parameters on the air turbulence was analyzed through the transition zone. The results showed that the local airflow characteristics and therefore the SCPP efficiency were significantly impacted by changes in junction radius. Besides, the maximum velocity inside the chimney was varied and changes its location when the junction radius changed. These facts affect directly the overall turbine expenditure, which represented in terms of structure and power capacity.