Online monitoring of power semiconductor devices is essential for ensuring the reliable operation of power electronic systems by providing critical information. In this article, a thermal resistance estimation method based on the optimized Miller voltage measurement circuit for IGBTs (insulated gate bipolar transistors) is proposed. The existing Miller voltage measurement method was optimized, resulting in an 80% reduction in measurement error. Furthermore, it was demonstrated that the Miller voltage is not affected by device aging. Building upon this, a thermal resistance estimation method for IGBTs is proposed, and experimental results validate the reliability of the proposed method. In comparison to the results obtained from the dual interface test, the proposed method exhibits relative errors in the estimation results of less than 2% under various load conditions, thereby validating the accuracy of the measurement method.