Many solar technologies are developed to encourage the usage of solar energy throughout the world. The solar chimney power plant (SCPP) is a newfangled technology that utilizes solar energy to generate more clean energy. This paper presents a new design of the SCPP turbine based on the aerodynamic optimization approach of a wind turbine with a diffuser. To calculate the required design parameters, a comprehensive mathematical model has been developed, which has the ability to describe the flow inside solar chimneys. An iteration algorithm is implemented for solving blade element theory with a diffuser to obtain the optimization of the turbine blade chord and twist angle distributions in the presence of a diffuser. The comparison of the mathematical model results and experimental data of Manzanares prototype reveal the good agreement, validating our mathematical model. The present design suggests that the power output could be increased by 15%. Our conclusion is that the blade element theory with diffuser has an ability to design efficient turbine suitable for working within SCPP.