Abstract: Nitrogen is an essential nutrient for crop growth and plays an irreplaceable role in ensuring global food production. However, the excessive application of nitrogen fertilizers in agricultural fields is widespread, and nitrogen use efficiency remains generally low. A large amount of nitrogen is lost through pathways such as NH₃ volatilization, N₂O emissions, and NO₃⁻-N leaching, which not only wastes fertilizer resources but also exacerbates a series of ecological and environmental problems, including water eutrophication, soil acidification, and greenhouse gas emissions. Under the strategic requirements of food security and the “dual carbon” goals (carbon peak and carbon neutrality), reducing nitrogen fertilizer losses while stabilizing or increasing crop yields has become a research hotspot in the field of sustainable agricultural development. This paper first introduces the key processes of soil nitrogen transformation in paddy fields, and then systematically reviews the effects and mechanisms of nitrification inhibitors and urease inhibitors on the major nitrogen loss pathways in paddy fields. Nitrification inhibitors delay the conversion of ammonium-N to nitrate-N by inhibiting the activity of ammonia−oxidizing microorganisms, thereby significantly reducing N₂O emissions and nitrogen leaching, but may increase the risk of NH₃ volatilization due to elevated soil ammonium-N concentrations. Urease inhibitors delay urea hydrolysis by inhibiting urease activity, effectively reducing NH₃ volatilization and nitrogen leaching; however, their mitigation effect on N₂O emissions shows substantial spatial and temporal variability. Meanwhile, this paper analyzes the effects of the synergists on rice growth and yield. Moderate addition of synergists can maintain a high ammonium-N environment in the soil, promoting rice tillering, enhancing photosynthetic efficiency and root activity, thereby increasing yield. However, excessive addition or specific soil conditions (e.g., Gleyi-stagnic anthrosol) may limit yield or even cause toxic effects. Furthermore, this paper discusses the application of biological nitrogen synergists, as well as innovative applications of synergists in new cropping patterns such as ratoon rice and rice-crayfish co-culturing. Finally, future research directions are proposed, including the development of diversified and blended synergist products, investigation of the synergistic mechanisms of nitrogen transformation and emission reduction with carbon sequestration under mechanical deep−application conditions, and comprehensive life-cycle environmental impact assessments that include indirect emissions such as NH₃ volatilization.