Polarization-controllable high-order harmonics are an effective tool for exploring dichroism in photoelectron spectroscopy. They are extremely useful for examining the ultrafast dynamics processes, electronic state symmetry and topology, and evolution of electronic structures of chiral substances and quantum materials after photoexcitation. This study focuses on the technical solutions for controlling the polarization of high-order harmonics, analyzes two types of methods for characterizing the polarization of high-order harmonics in spectroscopy and electron spectroscopy, introduces the research progress and achievements of circularly polarized high-order harmonics in applications to atoms, molecules, and quantum materials, and summarizes and predicts the future development directions of polarization-controllable high-order harmonics.