Ac-Stark shift of atom levels is caused by an ac-electromagnetic field.As an electromagnetic wave,laser light does induce ac-Stark shift.It is proved experimentally that if the light is linearly polarized,the dynamic polarizability changes with polarization direction.The polarization direction of the linearly-polarized laser is tuned by 720°,and the ac-Stark shifts of the 4S1/2,m=±l/2 →3D5/2,m=±l/2 clock transitions in 40Ca+ are measured in steps of 10°.The frequency shifts change with laser polarization in a periodical manner and have values opposite to each other.