Magnetronlike structure is a non-cathode magnetron waveguide structure. It is the basic configuration of relativistic magnetron with axial extraction (MDO). First, the dispersion relation and the field expression of the magnetronlike structure are obtained by the field matched method. Then, the results are proved by the high electromagnetic field analysis software, and the cutoff frequency is studied by changing the structure parameters. The final results show that with keeping the other parameters constant, the π mode cutoff frequency can be promoted by reducing the outer radius, enlarging the inner radius or increasing the number of the cavities. Enlarging the cavity angle will reduce the π mode cutoff frequency. The outcomes produced in this paper will play a fundamental role in the theoretical analysis on the mode conversion and transmission of the relativistic magnetron with axial extraction.