Online monitoring of junction temperature for insulated gate bipolar transistors(IGBTs) is an important basis for life prediction and health management of power electronic devices. To this end, an optimized online monitoring method of IGBT junction temperature based on switch overshoot voltage is proposed in this paper. Firstly, the expression of overshoot voltage is analyzed, and the online monitoring model of turning-on overshoot voltage is proposed. Secondly, a sampling circuit of the online monitoring model is analyzed and designed, and the parameters of sampling circuit is calculated. Finally, the experiment is carried out to show that the proposed online monitoring method of IGBT is feasible in practical applications and has high accuracy.