With the rapid advancement of laser technology, ultrafast optics has become a very important frontier area in modern physics research. High-harmonic generation, a primary technique for generating ultrashort laser pulses, has progressed rapidly in the past decade. In this paper, the conservation of spin angular momentum and orbital angular momentum, the spin-orbit interaction, and other novel physical phenomena in the high-harmonic generation in the gas medium, are reviewed. In addition, gaps and challenges in existing research are summarized. The application of structured light field in high-harmonic generation significantly enhances investigations of light-matter interaction and creates new opportunities in light manipulation and strong-field physics fields.