Para-xylene(PX)is an important chemical raw materials. In order to study the influence of external electrical field on molecular structure and infrared spectra of PX, the method B3LYP of the density functional theory at 6-311++G(d, p) level has been used to calculate geometrical parameters and infrared(IR) spectra under different external electric fields ( from 0 to 0.030 a. u.) in this article. The results show that the most strongest absorption of IR spectra of PX is produced by C11-H14 and C15-H17 stretching vibration; the molecular geometry parameters is strongly dependent on the external field intensity; the significant negative (“red”) and positive (“blue”) frequency shifts, the redistribution of molar absorption coefficient are observed, i. e., vibrational Stark effect(VSE) is obvious.