On the basis of our previous elastic model which is related to the salt ion conc entration, we put forth a new model of B-DNA to Z-DNA transition by considerin g interactions of the hydrogen-bond and base-stacking in Z-DNA, and discuss the effects of salt concentration on the structure transition of B-Z and obtain the force extension curves, the probability distribution curve of the folding angle and the critical torque of B-Z transition at various salt concentrations. The results show that Z-DNA appears when salt concentration reaches 2.4mol/L,and the probability of Z-form begins to increase and the torsion of B-Z trans ition decreases with the increase of salt concentration.