The ongoing evolution of power semiconductor devices toward higher power density, increased switching frequencies, and enhanced integration has introduced critical thermal management challenges, severely compromising their operational stability and long-term reliability. Silicon nitride (Si3 N4 ) ceramics, which are distinguished by exceptional thermal conductivity, superior mechanical performance, and a coefficient of thermal expansion (CTE) that is compatible with third-generation semiconductors, have emerged as ideal candidates for high-performance ceramic substrates. The composition and corresponding ratios of sintering additives strongly affect both the sintering process and final properties, making the selection of optimal additives a difficult challenge in realizing Si3 N4 ceramics with excellent heat conduction and robust mechanical reliability. This review systematically summarizes the current research progress in oxide and nonoxide sintering additive systems for the comprehensive fabrication of high-performance Si3 N4 ceramics and analyses the mechanisms by which various additive systems tailor the densification process, microstructure evolution, thermal conductivity and mechanical properties. Moreover, it forecasts future development trends and research directions for sintering aid systems for Si3 N4 ceramics with high thermal conductivity and high strength.