Wet granular column collapse is a generalized experimental model reflecting wet geophysical flows widely observed in nature, involving complex coupling among solid, liquid, and gas phases. However, the dynamic mechanisms are still not understood. Here, laboratory experiments on granular column collapse under dry and wet conditions are conducted in an acrylic flume with different particle sizes and aspect ratios. The deposit morphology and mobility of the collapsed columns are determined and compared. To further investigate the dynamic behavior of wet granular flow, a DEM model considering capillary and viscous forces between particles is developed and calibrated according to the experimental results. The dynamic characteristics and underlying mechanisms of granular column collapse are systematically investigated through experiments and numerical simulations. The results show that interstitial water restrains granular flow mobility, and a decrease in the particle size causes a significant increase in the restraining effect and a transition in the deposit morphology. The evolution of energies proves that capillary forces are critical in wet granular flows. The underlying mechanisms for the differences in morphology and mobility are also revealed by the collapse pattern characterized by particle temperature and the flow regime characterized by the Froude number.