Fluid-like motion and linear wave propagation behavior will emerge when we impose boundary constraints and polarized conditions on crowds. To this end, we present a Lagrangian hydrodynamics method to simulate the fluid-like motion of crowd and a triggering approach to generate the linear stop-and-go wave behavior. Specifically, we impose a self-propulsion force on the leading agents of the crowd to push the crowd to move forward and introduce a Smoothed Particle Hydrodynamics-based model to simulate the dynamics of dense crowds. Besides, we present a motion signal propagation approach to trigger the rest of the crowd so that they respond to the immediate leaders linearly, which can lead to the linear stop-and-go wave effect of the fluid-like motion for the crowd. Our experiments demonstrate that our model can simulate large-scale dense crowds with linear wave propagation.