Reasonable dead time is a prerequisite for realizing zero-voltage turn-on(ZVS) of IGBT in a three-phase LLC converter. At the same time, the operating temperature greatly affects the output capacitance characteristics of IGBT. When the operating temperature of the IGBT rises, it will result in the dead time that has been set is no longer appropriate, thus making the IGBT lose the soft-switching operating characteristics. To address this problem, the article analyzes the soft-switching realization conditions of the three-phase LLC converter and studies the relationship between the soft-switching realization and the dead time. The shutdown characteristics of IGBTs at different operating temperatures are analyzed. It is found that the higher the operating junction temperature of IGBTs in three-phase LLC resonant circuits, the more unfavorable it is to realize soft switching, and the formulas of the minimum dead time and the maximum dead time to ensure the zero-voltage turn-on of IGBTs at the worst operating temperatures are deduced. A 100 kW three-phase LLC converter prototype is constructed for verification. The soft switching can still be realized and the high efficiency can be maintained under the case of higher operating power, which verifies the accuracy of the dead time optimization design.