Abstract: In order to make the output impedance of general instruments match with the characteristic impedance of coaxial conical, and then reduce the echo reflection, a matching section is designed. S parameters with three different kinds of impedance gradient matching sections such as index gradient line, triangle gradient line and klopfenstein gradient line are compared respectively. VRSW of matching sections are simulated respectively with changing inner conductor or outer conductor, the results show that the matching section of impedance index gradient with inner conductor changing has a good characteristic with standing wave less than 1.2 under 10GHz, less than 2.5 under 20GHz.