A method aimed at dielectric parameters measurement of small samples using a coaxial re-entrant cavity is proposed. After introducing grooves in the cavity structure to suppress parasitic modes, the operating frequency range has been broadened to 0.6-13 GHz for only one cavity. Compared to the traditional structure, the working bandwidth has expanded to 344% of the original one. The cavity perturbation technique is applied to detect the complex permittivity. The proposed method features two advantages over existing techniques, high sensitivity and more convenience for placing the sample.