Over the last decades, manipulating polarization has received much attention due to its wide applications in science and technology. In this paper, a half-waveplate based on a field transformation (FT) method is proposed and investigated in order to convert polarization, which works at millimeter-wave band with a wide incident angle and broad working bandwidth. Inspired by the FT method, we confine our attention to a two-dimensional (2D) case of in-plane wave propagation on the x-y plane, with both material properties and fields unchanged in the z direction. The fields are denoted with a subscript “(0)” in the virtual space. Then a series of theoretical calculations is analyzed in detail. Under the guidance of theoretical analysis, it is shown that the main job for realizing this half-wavepalate is to obtain a material with specific permittivity and permeability. The proposed waveplate is composed of periodically arranged two dielectric layers each with sub-wavelength in height. By using the effective medium theory, the effective electromagnetic (EM) parameters of the waveplate can be tuned by manipulating the heights of the two dielectric layers. Among them one layer is a material with a permittivity of 10 and height of 0.68 mm, and another layer material has a permittivity of 1, and height of 5 mm. We alternately arrange the two materials along one direction periodically to obtain a waveplate for realizing the TE-to-TM and LCP-to-RCP conversion. The thickness of whole waveplate is 5.5 mm. A broadband EM half-waveplate is achieved in millimeter-wave region, which possesses a nearly 90% conversion efficiency across the frequency band from 24 GHz to 37 GHz. At the same time, we also find that when the incident angle gradually increases from 0° to 60°, the performances of polarization conversion efficiency and working bandwidth are still good. For the incident angle of 60°, a 3-dB bandwidth over 26-33 GHz is still achieved. The performance of the waveplate is verified through both full-wave simulation and experimental measurement, which are in good agreement with each other. Meanwhile, three-dimensional (3D) printing technology makes the sample fabricated more easily. Another advantage of our design is that the 3D printing technology can be used to carry out the experimental fabrication, which may pave a new way to manufacturing more microwave devices.