Forbush decrease (FD) event is one of the most important short-term modulations of galactic cosmic rays (GCRs) caused by intense solar activities such as interplanetary coronal mass ejection (ICME). The modulation mechanisms of GCRs by the disturbed interplanetary magnetic fields (IMF) of ICME and the accompanying forward interplanetary shock (IP) are not clear yet. In this work, we present a one-dimensional dynamic model of the GCR barrier driven by ICME. In our model, the time dependent radial diffusion coefficient rr of GCRs is depressed to be (r)rr (0 (r) 1) as they run into the disturbed IMF. The scale factor (r) is inversely proportional to the local solar wind speed away from the Sun. Within the disturbed area at any time, (r) increases exponentially from the local minimum (rsh)) at the IP front to 1 at the end of the ICME tail. In addition, (rsh)) switches gradually from its global minimum m at the bursting of the CME to 1 as the shock moving toward the outer boundary of the heliosphere. The geometrical and dynamic parameters of the ICME and IP are derived from the observations of GOES and ACE satellites. Based on the stochastic transport theory, the one-dimensional backward stochastic differential equation (SDE) method is adopted to simulate the transport of GCRs modulated by single halo ICME. The evolution of the neutron flux at the ground is calculated according to the recently reported proton-neutron yield function. As an example, the FD event on 15 May 2005, caused by the CME event bursting on 13 May 2005, is studied and simulated. The results show that the calculated neutron flux evolution, including not only the main and recovery phases, but also the pre-enhancement before the arriving of the CME at the Earth, is consistent with the observation of Oulu neutron monitor. According to the trajectories of GCRs, it can be found that, the per-enhancement of the neutron flux is a result of the scattering by the forward IP passing 1 AU. Before the IP reaches the switch cutoff Rc, GCRs are evidently confined in the sheath between the IP and CME. After that, the GCRs will stay for longer time in the magnetic cloud of the ICME as a result of the damping of IP strength. The parameterzed one-dimensional GCRs modulation model and the SDE method, as have been confirmed by the neutron monitor observation on the Earth, can be used further to calculate and predict the GCRs fluxes of other places, such as the Mars, in the heliosphere.