Agricultural plastic mulch films, while enhancing crop yields, also cause severe “white pollution.” As an environmentally friendly alternative, biodegradable mulches (BDMs) have attracted significant attention in recent years. This paper systematically reviews research progress on the environmental behavior of BDMs in soil, with a focus on analyzing their degradation mechanisms, the release characteristics of microplastics and additives, and their associated environmental risks. BDMs can be degraded through the synergistic action of chemical hydrolysis, photo-oxidation, and microbial enzymatic breakdown. However, the degradation rate is influenced by environmental factors such as soil temperature, moisture, and microbial communities. The degradation process of BDMs typically leads to a short-term, concentrated release of microplastics, with abundances considerably exceeding those from traditional plastic films. These microplastic fragments can further adsorb other pollutants. BDMs additives are characterized by their diverse types and high actual leaching potential in soil, yet their actual toxicity risks remain unclear. Current research on degradation mechanism, pollutant release and environmental risks of BDMs is still insufficient. Future efforts require material innovation, in-depth analysis of environmental behavior, and policy coordination to optimize the degradability of BDMs and reduce ecological risks.