We calculate the vibrational properties of potassium-doped single-walled carbon nanotubes within lattice dynamical theory. The results show that the frequency of high-frequency Raman mode E2g for K5C40 downshifts to 1553cm-1, which is in agreement with the value for highly doped samples with effective composition KCs.Moreover, the specific heat curves have a tuover at 22 K, originating from the saturation of K atoms vibrational modes at low energy.