Novel magnetically insulated transmission line oscillator with 2D periodic slow wave structure is put forward, the symmetrical mode and dissymmetrical mode of which have the same working frequency. It can get microwave with pure frequency during mode competition. At the same time the efficiency is increased. 3D electromagnetic software indicate that the symmetrical mode and dissymmetrical mode of slow microwave structure of the new magnetically insulated transmission line oscillator(MILO) have the same π mode resonance frequency while PIC simulation indicate that high-power microwave with muti-mode and pure frequency of 1.22 GHz is generated and the average output power is increased to 2.8 GW from 2.4 GW compared with the original MILO when the beam voltage is 450 kV and current is 43 kA.