The populations of n=17,m=0 Stark states of sodium atom in static electric fields have been obtained by two step excitation using tunable lasers. The lifetimes of some selected Stark states and the influences of blackbody radiation on the lifetimes were measured by using delayed electric field ionization. The measured data were compared with the calculated values. The experimental and theoretical results exhibit following behaviours: The total rates of stimulated emission and absorption transitions induced by environmental radiations are of the same order of magnitude in comparison to those of spontaneous transition for the observed n=17 Stark manifold states at room temperature. And this effect is more remarkable for longer lived states. It, along with the effect of the Stark mixing of l sublevels, will reduce the differences among lifetimes of Stark states in a manifold.