Establishing the coupling mechanism of hot carriers and phonons is at the heart of nanoelectronics and photocatalysis of vdW heterostructures. Herein, via ab initio simulations, we endeavor to examine the intralayer phononic assisted recombination in a designed Z-scheme vdW heterostructure based on an emerging WSi₂N₄ material and transition metal dichalcogenides. Through integrating WSi₂N₄ with MoS₂ (or its isoelectric cousin MoSe₂), an intralayer phonon assisted electron–hole emission is revealed via nonadiabatic molecular dynamics calculations. Accompanied by weaker intralayer phonon modes, the WSi₂N₄/MoS₂ interface shows a weaker nonadiabatic coupling and a longer electron–hole recombination time of up to 9.41 ns than the WSi₂N₄/MoSe₂ heterolayer. Moreover, the staggered and appropriate band alignment in WSi₂N₄/MoS₂ suggests its promising applications in photocatalysis such as water splitting. By comparing MoS₂ and MoSe₂, our time-domain simulations suggest that vdW heterostructures with a facile intralayer sliding and softening intralayer phonon would be beneficial for prolonging the lifetime of carriers.