Microwave dielectric ceramics are key materials for manufacturing 5G communication components. The microwave dielectric ceramics of BaSn(Si1-xGex)3O9(0≤x≤1.0) were prepared via conventional solid-state reactions, and the effect of substituting Ge4+ for Si4+ of BaSnSi3O9 ceramic on the sintering behavior, crystal structure, and microwave dielectric properties was investigated. The results show that the BaSnSi3O9 ceramic exhibits the porous microstructure and poor microwave dielectric properties (i.e., εr=6.61, Q×f=7 977 GHz at 15.03 GHz, and τf=-37.8×10-6/℃). The BaSn (Si1-xGex)3O9(0≤x≤1.0) solid solutions are obtained with Ge4+ substitution for Si4+, the crystal structure is a hexagonal structure with P-6c2 space group. The Ge4+ substitution for Si4+ can improve the microwave dielectric properties of BaSn(Si1-xGex)3O9(0≤x≤1.0) ceramics by optimising the sintering behavior and changing the crystal structural parameters. The Q×f values of BaSn(Si1-xGex)3O9(0≤x≤1.0) ceramics are mainly controlled by the relative covalency of Si/Ge-O and Sn-O bonds. As x=1.0, the BaSn(Si1-xGex)3O9 ceramic exhibits the optimal microwave dielectric properties (i.e., εr=8.53, Q×f=15 829 GHz at 14.41 GHz, and τf= -34.2×10-6 ℃-1).