High thermal conductivity Si3N4 ceramics with excellent mechanical properties were prepared by using ternary composite sintering agent Er2O3-Mg2Si-Yb2O3. The effects of Er2O3-Mg2Si-Yb2O3 system on densification, microstructure, mechanical properties and thermal conductivity of Si3N4 ceramics were studied. The results show that when the sintering agent is 5% (mass fraction, the same below) Er2O3+2%Mg2Si+4%Yb2O3, the effect of sintering agent on the balance between density and the content of grain boundary phase of Si3N4 ceramics is the best. At the moment, Si3N4 ceramics have the best properties, such as bending strength of 765 MPa, fracture toughness of 7.2 MPa·m1/2, and thermal conductivity of 67 W/(m·K). In the sintering process, adding only 5%Er2O3+2%Mg2Si produces less liquid phase with high viscosity, which cannot densify the Si3N4 ceramics. When the content of Yb2O3 exceeds 4%, the sintering agent produces a large amount of grain boundary phase, which reduces the properties of Si3N4 ceramics.