This article presents the study on the self-sustained oscillation of cascode gallium nitride (GaN) high-electron-mobility transistors (HEMTs), which occurs under the short-circuit (SC) condition. Based on the SC test, it is found that the self-sustained oscillation can be excited in the SC event of cascode GaN HEMTs. Moreover, the gate resistance R G does not have a significant damping effect on the self-sustained SC oscillation. The SPICE simulation is performed to study the oscillation waveforms of the internal depletion-mode HEMT (DHEMT) and low-voltage (LV) mosfet. This article reveals two positive feedback loops, which excite the SC oscillation. One loop is related to the positive feedback process of the DHEMT itself. Another loop is induced by the interaction between the DHEMT and LV mosfet. The two positive feedback loops are interrelated and can reinforce each other. As a result, a very strong driving force is generated to excite the self-sustained oscillation. During the oscillatory transient, the parasitic elements of the device's package play a critical role in exciting the positive feedback process. By analyzing the influence of the parasitic elements on the positive feedback process, the necessary methods are proposed to suppress the SC oscillation. The SPICE simulation validates the effectiveness of the proposed methods.