We systematically investigate the dynamical response of a three-dimensional kinetic isotropic Heisenberg spin system (ISS) to an exteal driving field by Monte Carlo numerical simulation. The dynamical response of the isotropic HSS differs obviously from those of the anisotropic Ising spin system in which no realstable ordered state and relevant dynamical phase transition were observed in the kinetic ISS. There is a threshold time after which the magnetization m(t) in the ISS driven by a symmetrical exteal field always tends asymptotically to a disorder state regardless of the initial state of the system. The threshold time depends on the reduced temperature T/TC of the investigated spin system, amplitude H0 and frequency of the exteal field, i.e. γ = C. ωα . H0-β. (T/TC)-γ.C is a constant equal to 0.0302 for a three-dimensional lattice and exponents а = 1.18 ± 0.01, β = 1.81 ± 0.01and γ = 1.68 ± 0.01.