In order to describe the diffusion behavior of low-pressure plasma, the low-pressure nonsteady diffusion fluid model is built using the equations of ion continuity and ion motion, Boltzmann's relationship of the electron, and variable mobility of the ion. The plasma recovery process in pulsed plasma is described by this model from the viewpoint of diffusion, which is the basic physical mechanism causing recovery. The fluid model is verified to be accurate compared with the particle-in-cell method. The characteristics of multipulse sheath dynamics are studied using this model for inner surface modification of a tube by the plasma-based ion implantation (PBII). Compared with the no-diffusion case, the sheath expansion during pulse-on time is accelerated, and the sheath is thicker when considering the plasma diffusion. During pulse-off time, the plasma recovery behavior of the ion-depleted region is obtained. For a shorter pulse-off time, the plasma cannot recover to its initial state. The maximum of the ion-implantation current can be strongly decreased due to the incomplete plasma recovery, but the average ion-implantation current is improved and achieves its maximum when the duty cycle is 0.8. All these results can provide beneficial theoretical guidance for the parameter optimization in the PBII.