The efficiencies of capping agents in immobilizing sediment phosphorus (P) are widely assessed using a short-term batch adsorption method. This is done based upon the assumption that the immobilization process can be simplified to an adsorption process. In this study, the immobilization efficiency of calcium-rich attapulgite (NCAP) for sediment P was assessed using both the batch adsorption and sediment core incubation methods. The techniques of high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) were employed to measure the soluble reactive P (SRP) and labile P in sediment cores, respectively. The results showed a high immobilization efficiency (on average, 95%) of the NCSP for SRP in the sediment during the 10-day batch adsorption, whereas 26% of Fe bound P and 48% of Al bound P were transformed into Ca bound P. In contrast, the capping of NCSP in sediment core caused a maximum of 145% increase in SRP concentration in the overlying water during the 110-day incubation. The concentrations of SRP and DGT-labile P exhibited increases of 176% and 193% in the top 30 mm and 20 mm sediment layers, respectively. No considerable transformation occurred from active Fe and Al bound P to stable Ca bound P. The contrasting results from NCSP amendment highlighted the difference in interactions between the NCSP and the sediments under the two assessment methods. As the incubation using sediment cores approached the field conditions, a long-term test based upon this method is strongly recommended before the costly field trial.