An Au nanoparticle irradiated by a femtosecond laser pulse was simulated with a modified molecular dynamics model. The local order parameter method was used to distinguish solid and liquid phase atoms and the melting characteristics of particles were obtained. The influence of different intensities of laser energy on the melting process was also explored. The results show that the spatial distribution of the gold atoms is transfered from the face centered cubic (FCC) to the loose arrangement of irregular disorder when the phase change occurs. It can be found that the melting process is a full scale melting, there is not an apparent interface between solid and liquid. And the proportion of melting increases gradually with the continuous laser irradiating.