Abstract: The phase transition of a DNA hairpin under a pulling force was investigated by using the two-dimensional (2D) Langevin dynamics simulations. It is found that the unfolded probability could be quantitatively expressed as a function of the pulling force, which is interpreted from the perspective of statistical mechanics. Then, by performing vast simulations with different parameter sets, the force-intensity of base pairing and the force-temperature phase diagram was plotted, respectively. It is shown that the increasing intensity of base pairing and the decreasing temperature are in favor of the folded states. The phase diagrams obtained in the present work are conductive to deepening understanding of the thermodynamic properties of a DNA hairpin under a pulling force.