This paper presents a modified Stewart platform that enables precise adjustment of the sub-reflector for the Qitai Radio Telescope (QTT). QTT demands a parallel platform capable of carrying a heavy load (≥4000 kg), and moving in elevation from 5° to 88° together with the primary reflector while precisely adjusting the sub-reflector's position in five degrees of freedom. To meet these requirements, a modified Stewart platform with two separated actuators is proposed, and the comprehensive performance is analyzed and optimized for practical application. The performance of the modified platform is compared to that of the traditional Stewart platform, and the results demonstrate that the modified platform has superior load-bearing capacity and stiffness over the entire elevation angle with more uniform actuator load and stiffness distributions. These results indicate that the modified Stewart platform is well-suited for practical application in QTT.