A new type of transmissive terahertz polarization conversion device is proposed based on the metamaterial structure of the combination of square open ring and metal rod. Specifically, the influence of different combinations of basic structural elements on the polarization conversion characteristics is first studied. The working bandwidth of the polarization conversion function is improved by optimizing the relevant parameters of the structural units. Finally, the efficient X-Y crosspolarization conversion is realized in the range of 0.45~1.30 THz, which is expected to be used in the research and application of integrated terahertz polarization conversion devices.