Microplastics (plastic particles smaller than 5 mm) pollution has become a global environmental research focus, with their accumulation in soil ecosystems—particularly agricultural environments—increasingly severe and posing serious threats to soil health and ecological functions. Although biodegradable microplastics (such as polylactic acid and polybutylene succinate) are considered green alternatives for mitigating “white pollution,” their degradation process may release more micro-particles, additives, and intermediate byproducts, exacerbating soil contamination. Moreover, they can affect soil carbon cycling through multiple pathways by altering soil structure, microbial activity, and enzyme functions. This review systematically summarizes the degradation characteristics of biodegradable microplastics and their mechanisms of influence on soil carbon cycling. After application to farmland, the degradation of biodegradable microplastics is regulated by material composition, environmental conditions, and microbial activity. The degradation products significantly increase soil dissolved organic carbon and promote CO₂emissions, while simultaneously reducing crop photosynthesis and carbon fixation efficiency. Compared to conventional microplastics, biodegradable microplastics may intensify short-term fluctuations in carbon cycling, but their long-term effects remain uncertain due to incomplete degradation and additive leaching. Future research should strengthen investigations into the long-term impacts of biodegradable microplastics on soil carbon cycling to provide scientific support for sustainable agricultural management and development.