Through numerical calculation, S-band magnetically insulated line oscillator (S-MILO) is calculated. The frequencies of the π/4, π/2, 3π/4 and π mode of primary coaxial slow wave structure (SWS) cavity calculated in the simulation are 1.40, 2.21, 2.46 and 2.51 GHz, respectrively. The frequencies of corresponding modes of the S-MILO closed cavity are 1.26, 2.04, 2.42 and 2.53 GHz. The quality-factor of π mode is also calculated. The eigen-frequency of S-MILO open cavity is 2.43 GHz by calculation, the frequency from 2.25 to 3.05 GHz is cut off in the RF choke, the output frequency is from 2.2 to 2.5 GHz. The results indicate that the high frequency structure of the S-MILO is reasonable.