Wideband metasurfaces usually have increased structure complexity as compared to their narrowband counterparts, by integrating multiple in-plane or cascaded resonant structures. We propose a dielectric grating structure as a quarter-wave plate with alleviated structure complexity and wide bandwidth at terahertz frequencies, by simply enlarging the period from subwavelength to superwavelength scale. From the modal theory, the superwavelength grating supports more than one waveguide modes. The superposition of the fundamental mode and higher-order mode with diverse dispersion characteristics is used to engineer the spectral response and to achieve a wideband linear-to-circular polarization conversion, which is validated by experimental study.