The invention belongs to the technical field of electronic information materials and components and aims to solve a problem that the dielectric constant of the ceramic materials in the prior art is unfit for producing a wireless passive temperature sensor in microwave resonator type. The invention provides a high-dielectric-constant microwave dielectric ceramic for a wireless temperature sensor and a preparation method thereof, wherein the microwave dielectric ceramic is in a bismuth-based cubic pyrochlore structure. The compound formula of the microwave dielectric ceramic is Bi<1.5+x>(Mg<0.8>Co<0.2>)Nb<1.5>O<7+1.5x>, 0.0 <= x <= 0.3. The preparation method includes the steps of synthesizing a nano powder through a sol-gel method, performing dispersion and granulation, filling a die with the granulated powder, performing bidirectional dry-press moulding on an oil pressing machine and performing plastic removal and normal-pressure sintering in a muffle furnace to obtain a ceramic block. The ceramic material is high in dielectric constant and the dielectric constant is sensitive temperature changes, and satisfies the requirement of principle of detecting environmental temperature achieved by means of a linear monotonous change relationship of the dielectric constant of the ceramic material to temperature with the wireless source-free temperature sensor in the microwave resonator type. The invention provides a microwave dielectric ceramic material which has huge potential in the field of wireless temperature sensors.