To reduce the size and improve the power density of three-level inverters, two or more associated power devices are assembled on the same heatsink, which, however, has a great influence on the thermal characteristics of heatsink because of heat accumulation, and is the main cause of thermal faults. The NPC (Neutral Point Clamped) three-level inverter was taken into this paper as a research target and platform. The dynamic power losses of two power devices were calculated firstly according to the working condition and then put into the solid model. Secondly, the thermal characteristics of heatsink with associated power devices on it were analyzed through finite element analysis, and the temperature rise of power devices were studied as well. Finally, the thermal characteristic experiment was conducted on a 1 MW NPC three-level inverter platform, and the results proved the effectiveness of the research method on thermal characteristics of heatsink with associated power devices.