Terahertz (THz) laser pulses can drive coherent vibrations through nonlinear phonon coupling, inducing transient lattice distortions that provide an optical pathway for manipulating quantum phases on ultrafast timescales. The excitation of specific phonon modes, in particular, shifts the atomic equilibrium positions, leading to structural distortions and symmetry breaking that alter the properties of symmetry-protected topological quasiparticles. Here, we investigate THz laser-induced nonlinear phonon interactions in the chiral semimetal CoGe, which contains multiple types of topological fermions, and reveal a symmetry-governed switching mechanism of topological states. Under THz excitation, lattice anharmonicity induces net displacements along the $A$ and $E$ Raman modes, breaking the $C_{3,111}$ rotational symmetry and lowering the space group from $P{2}_{1}3$ to its subgroup $P{2}_1{2}_1{2}_1$. Consequently, the unconventional chiral fermions with topological charges of 2 and 4 in CoGe transform into two and four Weyl fermions with the topological charge of 1 in the absence and presence of spin-orbit coupling, respectively. These findings demonstrate that ultrafast symmetry breaking driven by nonlinear phononics enables topological phase transitions of multifold fermions, offering a route for the dynamic control of topological quantum states.

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• Light-matter interaction
• Phonons
• Weyl semimetal
• Density functional theory
• First-principles calculations