Red mud is a highly alkaline solid waste with an annual emission of over 200 million tons, which requires large-scale utilization methods. Soil Cd remediation is a global concern, due to its high toxicity and strong mobility. Given red mud’s potential for soil Cd remediation, this study reviews its basic characteristics, the mechanisms of soil Cd immobilization by red mud, and the use of red mud-based passivators for agricultural soil Cd remediation. In general, red mud regulates soil pH, thus increasing the soil’s Cd adsorption capacity; provides abundant surface active sites for adsorption and complexation with soil Cd; introduces cations to immobilize Cd via ion exchange; and enriches Cd-resistant microbe species to reduce soil Cd toxicity. Furthermore, the potential environmental risks and suggestions on red mud application are discussed. Further research should focus on improving the remediation effectiveness of red mud on cadmium-contaminated agricultural soil, demonstrating its long-term efficacy and economic costs, and proposing practical technical models and standards for application.