The heat generating mechanism and dissipating feature research are carried out on the welded IGBT device in high voltage flexible DC converter valve. Based on the insulated gate biopolar transistor (IGBT) heat characteristics, the total thermal resistance is tested by transient hyperbolic method. The total thermal resistance corresponds with caculated results according to structural parameters and heat conductivity coefficients, which are derived from each layer of IGBT. This validates the heat conductivity coefficients, which are used for the calculation model by finite element method. Then, the integrated three-dimensional finite element model consisting of chip, substrate, bottom plate, silica GEL and shell is built. Steady-state thermal field, transient thermal field, heat and force coupling field are calculated successively. The steady-state thermal field shows that the selection of convection heat transfer coefficient of the bottom plate is the key to heat dissipation, and changing heat conductivity coefficient of silica gel has no obvious effect on the heat dissipation. From the transient thermal filed, the temperature reaches steady state after 28 seconds, and the highest temperature is 80.2 ℃. The thermal uniformity of each chip is good. The result of steady-state thermal filed is used as load to carry out the heat and force coupling calculation. The warp and residual stress results show that IGBT operates reliably. The heat dissipation of IGBT device is analyzed in detail by means of test and simulation, which provides technical support for the optimal design and reliable operation subsequently.