Modern weaving machines have the beat-up mechanism whose front position is predetermined mechanically. Therefore the relative position between the reed front and the clothfell plays an important role in producing the high quality fabrics with uniform weft density distribution. This research reports on the results from the simulation that was conducted on the basis of the theoretical model about the fabric weft density dynamics, including the factors such as beat-up motion and warp tension. Results showed that the weft density change was very sensitive to the initial clothfell position. The clothfell position was relocated toward the machine front during the machine restart and then it approached the steady state value. A low initial warp tension led to a narrow pickspace, too. Even though the warp tension and the beat-up force changed for a long period of time after the machine restarted, the weft interval variation and the clothfell position change were stabilized early, while the clothfell position showed an overshooting behavior for a low initial warp tension under a high level of process tension condition. Initial weft interval condition showed no influence on the dynamic behavior of the weft density.