Abstract: Atmospheric duct can change the normal propagation characteristics of electromagnetic wave, so it has been paid great attention by the academia. In response of the lack of the RSHMU model′s prediction performance Microwave power modules usually work under pulsed condition. When the pulse width time is larger than the response time of the chips, the traditional computational method of the transient thermal resistance based on the principle of linear superposition may cause large deviation. According to the characteristics of the even and repeated pulses of the microwave power modules, combining the transient temperature rise curve, we propose a novel computational method of the transient thermal resistance by solving an implicit equation. It shows from the comparison that the accuracy of the proposed method is higher than that of the traditional method under all pulse width range. This is more obvious especially under wide -pulse-width condition. The proposed method has a wide applicability, which can be applied in computing the transient thermal resistance of other pulsed electronic equipment, e. g. , the laser device and power supply, and etc.