Abstract: Cropland ecosystems are terrestrial ecosystems that are highly disturbed by human activity. Fertile soil, organic matter, and balanced nutrients play fundamental roles in ensuring crop growth, harmonious symbiosis of soil organisms, and stability of ecosystem productivity. In cropland ecosystems, crop straw and livestock manure are two main exogenous organic materials that are incorporated into soils; and are the main sources of soil organic matter. The incorporation of organic materials into soils can increase soil organic carbon accumulation, improve soil fertility, and enhance crop yield. Organic materials are ultimately transformed into soil organic matter (SOM) after undergoing a series of complex physical, chemical, and biological decomposition processes. These complex decomposition processes are controlled by many factors, such as organic materials properties, soil types and their inherent characteristics, environmental conditions, and agricultural management practices. Therefore, these processes and their driving factors are receiving increasing attention worldwide. This article comprehensively reviewed the main research progress in the decomposition characteristics of organic materials and their main influencing factors. The effects of organic amendments on soil nutrient pools and soil quality and the underlying mechanisms regulating the transformation of organic materials to soil organic matter were also discussed. This study first outlined the main methods and indicators for characterizing the decomposition processes of organic materials, analyzed the characteristics of the chemical structure of organic materials and soil during decomposition, and revealed the main factors regulating the decomposition processes of organic materials (environmental factors; the quality, composition, and chemical structure of organic materials). Second, the contributions of organic material incorporation to the soil organic carbon pool, active organic carbon, humus composition, nutrient pools, and soil quality were evaluated. Effects of incorporating organic materials on soil fertility were discussed in terms of crop yield and soil physical, chemical, and biological properties. Third, the underlying physical, chemical, and biological mechanisms regulating the transformation of organic materials to soil organic matter were illustrated. Finally, prospects for future research on the processes and mechanisms of organic material decomposition in the cropland ecosystems were presented, as listed in the following points: to optimize scientific and reasonable measures for organic materials returning to the field by considering local conditions, long-term positioning field experiments and cross-regional network experiments. The optimal ratio between the labile and recalcitrant components of organic materials must be quantified to achieve a highly efficient decomposition degree of the organic materials. More advanced and elaborate techniques are advocated to be applied together to characterize the decomposition products more comprehensively and accurately. This review is expected to provide a theoretical basis and data support for interpreting the decomposition and transformation processes of organic materials and optimizing cropland management practices in cropland ecosystems.