With the new energy connected to the grid, power quality problems have been focus by electricity setter and electricity parties, and the power electronics also have been more and more widely regarded. A junction temperature acquiring system of the 3L-NPC inverter base on Labview and AVR32 has been researched and designed in this paper. Compared with some traditional methods, The system has a higher accuracy and faster measurement speed. Firstly, the switching losses models are set up based on the analysis of working state of 3L-NPC inverter. And then, the junction temperature acquiring system has been designed and tested, the results verify the validity and effectiveness of the system.