This article addresses the challenge of achieving extended pulse widths in high-power microwave sources operating in long-pulse accelerators. Through modifications and improvements to the transit-time oscillator (TTO) structure, including a high- ${Q}$ modulation cavity, angled corners, a four-cavity extraction structure, and an internally extracted microwave output, the device’s pulsewidth was successfully increased from 42 to 68 ns. Experimental results confirmed the effectiveness of these enhancements, enabling the generation of a 1.03 GW microwave output at a frequency of 12.4 GHz. This study provides valuable experimental results for the development of high-power microwave sources and serves as a reference to advance research and applications in related fields.