Abstract: A solid-state transmitter，with 18 power amplifiers and the total heat power of 11.88 kW，can only be cooled by forced air cooling because of the limited volume．In this paper the thermal state is analyzed in theory and the cabinet and the power amplifiers are modeled with the software Icepak．By the function of Zoom-in，the power amplifier is modeled again in detail．On this basis，the peak temperature of the power amplifier is obtained in the cabinet．The maximum temperature is controlled within the required range by parameter optimization．The practice shows that the thermal design of the transmitter is successful and satisfies the requirement of the user．It also proves that the Zoom-in function of the software Icepak can reduce the number of the grid and improve the computation efficiency，with the precision ensured．Evidently it has practical value．