Silicon nitride (Si₃N₄) ceramics are a novel category of structural ceramics, exhibiting both high reliability and economy value due to their unique physical and chemical properties. However, their strong covalent bonds make densification challenging, and phase transition control remains an issue. In this study, high temperature and high pressure (HTHP) sintering was utilized in combination with MgO-Y₂O₃ binary additives (mass ratio Si₃N₄∶MgO∶Y₂O₃ = 94∶3∶3) to achieve a synergistic sintering process involving both high pressure and liquid-phase formation. The effects of binary sintering additives on the sintering process, phase transition behavior, microscopic morphology and mechanical properties of Si₃N₄ under high pressure were systematically investigated by designing a two-layer comparative experimental assembly to ensure the same sintering temperature. The results show that the MgO-Y₂O₃ promotes the α→β phase transformation of Si₃N₄ through liquid phase formation, which reduces the phase transition onset temperature from 1800 ℃ to 1650 ℃. Furthermore, high pressure enhances grain rearrangement and densification. As a result, highly dense Si₃N₄ ceramics with an optimal Vickers hardness up to (24.5±1.9) GPa were successfully synthesized. These findings provide an effective strategy for preparing high-performance Si₃N₄ ceramics and hold significant implications for advancements in physics and material science.