The invention discloses a device and a method for testing high-temperature complex permittivityes of low-loss dielectric materials, which belongs to the micro-wave, millimeter wave low-loss dielectric materials complex permittivityes testing technique. The testing device comprises a micro wave signal source, a cylinder type high Q resonant cavity, a scalar network analyzer, and a vacuum high-temperature furnace, wherein the cylinder type high Q resonant cavity comprises a cylinder cavity tube, an upper end cover, a lower end cover, and a connecting wave guide, whole resonant cavity adopts thin-wall precious metals to form a cavity, and adopts high-temperature resisting material to form a supporting body. Adoption of the testing device comprises testing a resonating frequency rate of an empty cavity f0 and a non-loading quality factor Q0, and testing the resonating frequency rate f0 epsilon and the non-loading factor Q0 epsilon of a rear resonating cavity of a testing sample of a loading low-loss dielectric under the same high temperature, and calculating out the high-temperature complex permittivityes of the low-loss dielectric materials. The invention has the advantages of wide applying frequency range, high testing temperature, low cost, convenient testing, and little testing error, and is applicable to high temperature tests of the complex permittivityes of the low-loss dielectric materials in each microwave, millimeter wave frequency band range.