Abstract: In this paper we have studied the excitation of plasma wake field by an ultrashort laser pulse in the near-resonant regime through one-dimensional particle-in-cell (PIC) simulations. Preliminary results are given of the amplitude of the plasma wake field versus laser pulse duration from resonant to near resonant regime. It is found that the wake field excitation is enhanced at near-resonant regime. The wake field is also obtained by numerically solving a nonlinear equation, which have been used to study wake field excitation under quasi-static approximation. By comparing the results from the two methods, the appropriate region of using the nonlinear equation is established that the laser pulse duration has to be 4 times less than the plasma wave wavelength. For longer laser pulses, the non