The cutoff frequency is soluted based on electromagnetic field theory, and network line is presented to analyze the relationships among coaxial connector’s cutoff frequency, the outer diameter of the inner conductor, the inner diameter of the outer conductor and the relative permittivity of the filled medium on the condition that impedance matched between coaxial connector and coaxial cable, which shows their relationships apparently. Finally, Three ways proposed so as to expand its working frequency band effectively. In addition, coaxial fixture at fixed size is studied in the field of electromagnetic parameters measurement, the result has shown that samples filled in the coaxial fixture are difficult to make the connector’s impedance exactly match with cable’s, and coaxial connector’s working frequency band reduces while the permittivity of test samples improved.