Computational fluid dynamics combined with discrete element method is employed to investigate the pressure signals and solid back-mixing behavior in a three-dimensional full-loop circulating fluidized bed operating in fast fluidization (FF) and dilute phase transport (DPT) regimes. The minimum fluidization velocity is successfully predicted after model validation. The gas solid full-loop hydrodynamics is accurately captured. Pressure signals under different fluidization regimes shed light on the flow dynamics. The wider solid residence time distribution (RTD) curve with a longer tail in the FF regime indicates that solid flow closes to perfect mixing flow, and more severe solid back-mixing is due to solid internal circulation existing in the riser. The smaller solid RTD curve with, a short tail in the DPT regime suggests that the solid flow deviates a little from plug flow) and a small-scale solid back-mixing is due to geometry restraint and recirculating gas-solid flow occurring in the lower and upper regions of the riser.