Safety, durability, and green low-carbon performance are key requirements for the development of highway slope engineering. Given the shortcomings of conventional masonry slope protection and vegetation-engineering composite systems with respect to construction duration, construction safety, and environmental impacts, this paper presents a systematic review of ecological protection technologies for highway subgrade slopes. The working mechanisms of typical ecological protection technologies, such as plant slope protection and attached-type slope protection, were systematically analyzed. Important Chinese and international research results on these technologies were summarized. The slope protection performance of different ecological protection technologies was discussed in depth, and future development trends of ecological protection technologies for highway subgrade slopes in China were prospected. Comprehensive analysis indicates that currently, there are no scientific and systematic guidance methods for plant selection and configuration in plant slope protection technologies; regarding vegetation blanket protection, the effects of blanket structural layer thickness, grid shape, and stitching pile arrangement on protection effectiveness require further in-depth study; the quality of ecological bags directly affects their protection effect, thus ecological bags with high water permeability and soil retention capabilities should be developed; the durability of ecological concrete and the potential impact of porous concrete on slope ecology require continuous research and evaluation; geocell slope protection faces problems such as geocell deformation caused by cultivated soil compression and water accumulation inside the cells during rainfall, which need to be solved urgently; for three-dimensional network plant protection technology, standards are lacking regarding material selection, net type selection, and substrate development, and research on the synergistic mechanism among plants, cultivated soil, and the three-dimensional network is insufficient. Currently, there are no unified standards for the design and evaluation indices of ecological protection technologies, and practical applications mostly rely on field experience. Therefore, future theoretical and application research should be conducted in the aforementioned areas to identify the most suitable ecological protection technologies to replace skeleton-plant slope protection, providing guidance for promoting the development of ecological protection technologies for highway subgrade slopes in China.