Microwave composite substrates are the key materials in the fields of aerospace, electronic countermeasure and 5G communication, etc., as they have combined high toughness of resin matrix and excellent dielectric and thermal properties of ceramic filler. Here, a new category of microwave composite substrates with polyphenylene oxide (PPO) as matrix and Ca_0.7La_0.2TiO₃ (CLT) ceramic as filler were prepared by a novel technology which combined screw granulating and injection molding. Its microstructure, microwave dielectric properties, thermal and mechanical properties of the substrates were characterized. The results show that such microwave composite substrates prepared by this technology maintain homogeneous composition and compact structure. As volume fraction of CLT ceramic increases from 0 to 50%, the dielectric constant of the substrate increases from 2.65 to 12.81 and the dielectric loss decreases from 3.5×10 ^-3 to 2.0×10 ^-3 (@10GHz). Meanwhile, coefficient of thermal expansion (CTE) of the substrate significantly decreases from 7.64×10 ^-5 ℃ ^-1 to 1.49×10 ^-5 ℃ ^-1 and the thermal conductivity increases from 0.19 W·m ^-1·K ^-1 to 0.55 W·m ^-1·K ^-1. Additionally, bending strength of the substrate is improved from 97.9 MPa to 128.7 MPa. The PPO/CLT composite substrate filling with 40% CLT (in volume) ceramic exhibits excellent properties: ε_r=10.27, tanδ=2.0×10 ^-3(@10GHz), α=2.91×10 ^-5/℃, λ=0.47 W·m ^-1·K ^-1, σ_s=128.7 MPa, so that it has good application prospects in aerospace, electronic countermeasure, 5G communication and other fields.