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The control of charge and spin degree of freedom of materials by ultrafast optical pulse enables novel sensing technology, quantum computing, terahertz emission, ultrafast optical switching, and ultrafast data storage. Chiral helimagnetic materials have recently attracted much attention for spintronic applications owing to their long-range helical magnetic order, topological spin textures, and potential for hosting skyrmions. Here, we present an ultrafast spin dynamics study in single crystals of chiral helimagnet Cr_1/3NbS₂ using the time-resolved magneto-optical Kerr effect technique. At low temperature and low external magnetic field, Cr_1/3NbS₂ exhibits a chiral helical magnetic phase with long-range helical spin order but contains a zero net magnetic moment. Due to the high uniaxial anisotropy of the crystal, a linearly polarized femtosecond laser pulse can magnetize the material in tens of ps, which brings the system into the chiral conical phase at zero-field and tilted chiral soliton lattice phase at a higher external magnetic field. In addition, we have found that the magnetic field-dependence of spin precession frequency in this helimagnet does not obey Landau-Lifshitz-Gilbert formalism as applied for ferromagnetic materials in general.