In this work, temperature stable microwave dielectric materials (1 − x)AMoO4–xTiO2 (A = Ca, Sr) were prepared by a solid state reaction method. The phase composition, sintering behaviors, microstructures, microwave dielectric properties, effective permittivity and vibrational phonon modes were investigated. The X-ray diffraction pattern and scanning electron microscope analysis indicated that the AMoO4 (A = Ca, Sr) phase could coexist with the TiO2 phase. The effective dielectric constants of the AMoO4–TiO2 composites were calculated by the finite element method (FEM), compared with the measured values and the numerical results obtained by the classical mixing rules. The correlation between the dielectric properties and the crystal structures were studied using IR and Raman spectroscopy. The infrared spectra were analyzed using the classical harmonic oscillator model, and revealed that the external vibration modes of AMoO4 (A = Ca, Sr) had the most significant influence on the dielectric constant. The Raman spectra showed that there were strong interactions in the [MoO4] tetrahedron due to the sharp and intense Raman modes. Finally, the low-firing (900 °C) microwave dielectric ceramics were obtained with 3 wt% H3BO3–CuO addition (BCu), and they possess good microwave dielectric properties with εr = 10.6–13, high Q × f values (40700–72050 GHz), and near-zero temperature coefficients of resonant frequency (TCF or τf values). These results also show that (1 − x)AMoO4–xTiO2–BCu (A = Ca, Sr) ceramics are good candidates for microwave electronic device applications.