The invention relates to a mode converter, in particular to a broadband frequency modulation coaxial plug board type mode converter with the high conversion efficiency. The broadband frequency modulation coaxial plug board type mode converter comprises a coaxial waveguide composed of an inner conductor and an outer conductor, and a first rectangular metal plate, a second rectangular metal plate, a third rectangular metal plate and a fourth rectangular metal plate of which the front ends are aligned are axially inserted between the inner conductor and the outer conductor of the coaxial waveguide; the mode converter is partitioned into a section A, a section B, a section C, a section D and a section E by the four metal plates from front to back sequentially; the third metal plate of the mode converter is composed of three metal plate bodies, and the middle metal plate body can axially move along with a groove formed in the inner conductor; two rows of support rods are arranged on the coaxial waveguide section A; the portion, in the section E, of the inner conductor extends from the portions, in the section A, the section B, the section C and the section D, of the inner conductor to the position flush with an outlet formed in the back end of the outer conductor. The mode converter has the wider conversion efficiency band width and the higher conversion efficiency.