The invention provides a system and a method for testing microwave complex permittivity of a dielectric material. The system comprises a coaxial-open-type resonant cavity and a coupling quantity adjuster. The method includes the steps: a, recording resonant frequency f0 of the system without any sample loaded in a testing range and a quality factor Q0 of a corresponding frequency point; b, testing thickness d of a to-be-tested sample, and testing to acquire resonant frequency f1 of the system after the sample is loaded in the testing range and a Q1 of a corresponding loaded quality factor; c, utilizing a perturbation theory of the resonant cavity to extract complex permittivity of a to-be-tested material. By the system and the method, high-accuracy testing of the complex permittivity of the microwave dielectric material is realized, and troublesome precise calibration does not need to be performed on a network analyzer, so that testing time is reduced, and requirements, on sample manufacturing, of testing are lowered.