The invention discloses a temperature-stable type low-dielectric-constant microwave dielectric ceramic Bi7Ti5O20C1and a preparation method thereof. The preparation method comprises the following steps: (1) weighing and preparing original powder of Bi2O3, TiO2 and BiOC1 with the purity of 99.9% (in percentage by weight) according to the proportioning of Bi7Ti5O20C1; (2) performing wet type ball-milling mixing on the raw materials in the step (1) for 12 hours, drying, and pre-sintering for 6 hours in the air atmosphere at 770 DEG C, wherein absolute ethyl alcohol is adopted as the ball-milling medium; (3) adding an adhesive into the powder prepared in the step (2), pelleting, pressing for forming, and finally sintering for 4 hours in the air atmosphere at 820-860 DEG C, wherein the adhesive is a polyvinyl alcohol solution with the mass concentration of 5%, and the polyvinyl alcohol accounts for 3% of the total mass of the powder. The ceramic disclosed by the invention is good in sintering property and has high application values in industry as the dielectric constant of the ceramic is up to 30.4-32.3, the quality factor Qf value of the ceramic is as high as 60,600-65,300 GHz, and the temperature coefficient of resonance frequency is small.