Utilization of recycled coarse aggregate (RCA) to produce recycled aggregate concrete (RAC) becomes an effective measure to dispose of the ever-growing construction and demolition waste. However, the high water absorption (WA) of RCA causes difficulty in controlling the fresh properties of RAC. In this study, different types of modeled recycled coarse aggregates (MRCAs) were prepared to investigate the effect of the quality and quantity of the attached mortar on the water-absorbing behavior (WAB) of RCA using an improved hydrostatic weighing method, as well as MIP and X-CT tests. The results showed that the WAB of RCA can be investigated by the designed MRCAs due to their similar WAB. The early WA and total WA of MRCAs were found to increase with the content and water-to-cement ratio (W/C) of the attached mortar, which can be explained by the higher pore content. There was a good correlation between the early-stage WA and 24 h WA, indicating that they can be used to predict each other. The pore size and distribution played a decisive role in the early WAB of RCA. The higher WA in the initial stage was attributed to the filling of large pores, while the gradually decreased WA indicated that the capillary WA of small pores started to dominate. The WAB of RCA can be explained by the hydraulic diffusivity model using the MIP result as the input parameter, which aligns with the continuous wetting condition.