The perturbation mechanism of the Galactic disk has puzzled us for a long time .
The imprints from perturbations provide important diagnostics on the disk formation and evolution .
Here we try to constrain when the vertical perturbation took place in the disk by tracking the phase mixing history .
Firstly , we clearly depict the spiral structures of radial ( v _ { R } ) and azimuthal ( v _ { \phi } ) velocities in the phase space of the vertical position and velocity ( z - v _ { z } ) with 723,871 LAMOST-Gaia combined stars .
Then , we investigate the variation of the spirals with stellar age ( \tau ) by dividing the sample into seven stellar age bins .
Finally , we find that the spirals explicitly exist in all the bins , even in the bin of \tau < 0.5 Gyr , except for the bin of \tau > 6.0 Gyr .
This constrains the vertical perturbation probably starting no later than 0.5 Gyr ago .
But we can not rule out whether the young stars ( \tau < 0.5 Gyr ) inherit the oscillations from the perturbed ISM where they born from .
This study provides some important observational evidences to understand the disk perturbation mechanisms , even the formation and evolution of our Galaxy .