A novel series of (1-x)MgGa2O4-xBaCu(B2O5) (x = 0.1–0.5) microwave dielectric ceramics were prepared by solid phase method. By adding BaCu(B2O5) additives, the sintering temperature of MgGa2O4 ceramics was reduced from 1410 °C to 925 °C. Through an analysis of the trend in packing fraction and FWHM value of T2g Raman peak, the underlying influence mechanism of various intrinsic factors on the dielectric loss of ceramic were revealed. And the density, as the external factors, exhibits a positive correlation with the dielectric constant and Q × f value. In addition, the high bond strength of Ga-site reduces lattice distortion and consequently enhances the frequency temperature coefficient. The microwave dielectric properties of (1-x)MgGa2O4-xBaCu(B2O5) (x = 0.3) are found to be superior at a sintering temperature of 925 °C (εr = 9.42, Q × f = 35,480 GHz, τf = −22.7 ppm/°C, at 14 GHz). Moreover, the chemical compatibility of ceramics cofiring with Ag electrode makes them a highly promising substrate for microwave communications.