Fast leading-edge RF pulse emission is an advantageous function of solid-state active phased arrays. In this paper, the conditions of realizing fast leading-edge RF pulse emission are analyzed, including influences and design points of RF excitation transmission, power amplification, timing synchronization and aperture fill time. The conclusions are applied to a prototype design of an X-band solid-state active phased array, followed by validation test. RF pulses emitted by thousands of solid-state active channels are spatially combined into one, with the leading-edge shorter than 5 ns, which validates the analysis.