A small amount of organic carbon, referred to as phytolith-occluded carbon (PhytOC), is occluded during phytolith formation. This carbon fraction has been demonstrated to be an important form of carbon sequestration. Thus, it has drawn particular attention from many researchers in the study of the terrestrial carbon cycle because it is of great significance for increasing terrestrial ecosystem carbon sinks and delaying global warming caused by the greenhouse gas effect. First, we briefly reviewed the research history and current status of PhytOC. Then, focusing on the long-term changes in PhytOC, we discussed the questions and challenges of PhytOC and terrestrial ecosystem carbon sinks as well as the recent argument that carbon sequestration by PhytOC in terrestrial ecosystems was highly overestimated. However, to more accurately estimate the phytolith carbon sink, some factors should be considered regarding long-term changes in PhytOC, such as different carbon-occluded abilities, carbon source, and stability of phytolith, decay rate of PhytOC in soil, and the impact of different phytolith extraction methods on PhytOC. As a result, the status and importance of PhytOC in studying terrestrial carbon sinks would be enhanced.