在DNA延伸过程中，考虑到氢键相互作用以及盐浓度(Na^{+)对氢键和堆积作用的影响，给出了修正的ZZO模型，得到了力-延伸曲线随盐浓度的变化规律。结果表明：随盐浓度的增加，延伸相变力非线性增加，并最终趋于稳定，理论计算结果与实验测量一致。}

We give the modified ZZO model of DNA, where the hydrogen-bond interaction and the effects of salt on hydrogen-bond and base-stacking interactions are taken into account. The force-extension curves are discussed at different salt concentrations. It is shown that the overstretching transition force increases nonlinearly when the salt concentration is increased. These results are in agreement experimental measurement.