We investigate theoretically the ionization properties of the valence electron for the alkali metal atom Na in an intense pulsed laser field by solving numerically the time-dependent Schr?dinger equation with an accurate l-dependent model potential. By calculating the variations of the ionization probabilities with laser peak intensity for wavelengths ranging from 200 nm to 600 nm, our results present a dynamic stabilization trend for the Na atom initially in its ground state (3s) and the excited states (3p and 4s) exposed to an intense pulsed laser field. Especially a clear"window" of dynamic stabilization at lower laser intensities and longer wavelengths for the initial state 4s (the second excited state) is found. By analyzing the time-dependent population distributions of the valence electron in the bound states with the different values of principal quantum number n and orbital quantum number l, we can attribute the dynamic stabilization to the periodic population in the low-excited states since the valence electron oscillates rapidly between the lowly excited states and the continuum states.