The rapid expansion of the biomass energy industry has led to a sharp increase in the production of ash and slag from biomass power plants. Traditional landfill disposal methods for these by-products pose high economic costs and potential environmental risks. However, biomass ash and slag are rich in essential plant nutrients such as calcium, potassium, phosphorus, and magnesium, and exhibit strong alkalinity, giving them considerable potential as soil amendments and ecological restoration materials. This offers a promising pathway for simultaneously promoting waste valorization and supporting the restoration of degraded ecosystems. A major constraint on their resource application is the potential presence of heavy metals (e.g., cadmium, chromium, lead) and other impurities, which, if not properly managed, can lead to secondary soil pollution. By integrating recent Chinese and international research with relevant policy documents, this review develops a practical framework for identifying suitable utilization scenarios, key environmental risks, and critical regulatory needs for biomass ash and slag. The paper systematically reviews the regulatory challenges involved in transitioning biomass ash from a “waste” to a “by-product” legal status, analyzes how its physicochemical properties are influenced by fuel source and combustion technology, and synthesizes resource utilization pathways in areas such as soil acidification amelioration, nutrient cycling, heavy metal passivation, and mine site restoration. While technical feasibility is supported by numerous studies, the establishment of unified quality standards, a scientific risk assessment framework, and a clear regulatory system—particularly for detecting and controlling impurities—is crucial for promoting the large-scale, high-value resource utilization of biomass ash and slag.