Recently, a high-efficiency thermoelectric candidate SnSe has been experimentally shown to undergo a phase transition induced by femtosecond laser excitation, but the underlying microscopic mechanism has not been fully clarified. Photoinduced phase transitions provide an effective means to disentangle electronic and lattice interactions on ultrafast timescales. Using real-time time-dependent density functional theory combined with occupation-constrained density functional theory methods, we investigate the structural response of SnSe under the excitation of 800 nm ($\hslash \omega =1.55\mathrm{eV}$). While the thermal fluctuations fail to destabilize the Pnma phase, promoting 5.4% of the valence electrons induces a rapid Pnma phase to Immm phase transformation. It is found that photoexcitation can reshape the potential-energy surface from hill-like to well-like and drive directional atomic motion. The dynamics are governed by several coherent phonon modes, such as the ${\mathrm{A}}_g^{\left(1\right)}$ and ${\mathrm{A}}_g^{\left(2\right)}$ modes, which play the dominant role in the phase transition. The transition process can be completed on a subpicosecond timescale, highlighting a carrier-driven, nonthermal mechanism for ultrafast phase control in SnSe. These findings offer valuable insights into regulating the phase behavior of SnSe-based systems under nonequilibrium conditions.

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• Electronic structure
• Phase transitions
• Semiconductor compounds
• Density functional theory