Flexible manipulation of chiral terahertz electromagnetic waves holds substantial potential for a wide range of applications, such as terahertz circular dichroism spectroscopy in biomaterials analysis, ultrafast electron bunch manipulation, high speed wireless communication and imaging. However, the development of tunable terahertz polarization modulation has been impeded by the lack of terahertz flexible manipulation measure at room temperature. Here, an innovative element based on patterned spintronic terahertz sources is demonstrated, which can achieve high efficiency and great flexibility in polarization adjustment. The contributory effect of built-in electric fields on chiral terahertz waves is experimentally revealed by arranging different periodical microscale stripes, and swift polarization switching between linear, elliptical and circular states is achieved by rotating ferromagnetic heterostructures. Notably, the ellipticity of the circle polarization state remains above 0.85 over a broadband terahertz bandwidth (from 0.74 THz to 1.66 THz). Furthermore, various polarization states dependent on geometry and azimuth angles provide insight into the physical mechanism of terahertz modulation by the built-in electric field. These findings contribute to the development of novel multifunctional terahertz devices, which pave a way to implement on-chip tunable terahertz polarization spectroscopy applications in biomedical detection and high-speed communication.