The invention discloses application of rock-salt-structured composite oxide Li2Mg2TiO5 as a microwave dielectric ceramic stable in temperature and low in dielectric constant and a preparation method of the rock-salt-structured composite oxide Li2Mg2TiO5.The preparation method comprises the steps of 1, weighing and compounding original powder of Li2CO3, MgO and TiO2 with the purity being 99.9% (weight percentage) or above according to the composition of Li2Mg2TiO5; 2, conducting wet type ball milling and mixing on the raw materials for 8 hours, wherein ethyl alcohol absolute serves as a ball milling medium, and after the raw materials are dried, conducting presintering for 4 hours in air atmosphere; 3, adding binder to powder obtained in step 2, conducting granulation, then compressing the powder for formation, and finally conducting sintering for 4 hours in air atmosphere at the temperature of 1200-1300 DEG C, wherein a polyvinyl alcohol solution with the mass concentration being 5% is adopted as the binder, and the adding amount of polyvinyl alcohol accounts for 3% of the total mass of the powder.The ceramic prepared through the method is good in sintering performance, the dielectric constant reaches 12.3-14.7, the quality factor Q*f of the ceramic is as high as 72,000-85,000 GHz, the temperature coefficient of the resonance frequency is low, and extremely high application value is achieved industrially.