Spinel-type microwave dielectric ceramics are widely applied in wireless communication due to its high quality factor and adjustable resonant frequency temperature coefficient. In this paper, MgO·nGa2O3 (n=0.975, 1.00, 1.08 and 1.17) spinel ceramics were prepared by pressureless sintering. The influence of stoichiometric ratio on the crystalline structure was studied by the XRD Rietveld whole pattern fitting method. The relationship between microwave dielectric properties and crystalline structure was explored through the bond valence theory model. The variation of relative dielectric constant (εr) of MgO·nGa2O3 ceramics is related to the lattice constant and ion polarizability. The quality factor (Q×f) decreases from 165 590 GHz to 109 413 GHz as n increases, which is affected by bond strength and the degree of cation order. The resonant frequency temperature coefficient (τf) is related to the crystal thermal expansion coefficient. The obtained MgO·0.975Ga2O3 ceramics possess excellent microwave dielectric properties: εr=9.69, Q×f=165 590 GHz (at 14 GHz) and τf=-7.12×10-6/℃.