Karst water is rich in calcium ions (Ca²⁺) and exhibits poor metal availability and low biodegradation efficiency. This study sought to analyze the effects and mechanisms of Ca²⁺ on lead (Pb) removal and absorption by Eichhornia crassipes (a floating plant common in karst areas). Moreover, the morphology and functional groups of E. crassipes in water were characterized via SEM, and FTIR. The results demonstrated that the removal rate of Pb in karst water (85.31%) was higher than that in non-karst water (77.04%); however, the Pb bioconcentration amount (BCA) in E. crassipes roots in karst water (1763 mg/kg) was lower than that in non-karst water (2143 mg/kg). With increased Ca²⁺ concentrations (60, 80, and 100 mg/L) in karst water, the Pb removal rate increased (85.31%, 88.87%, and 92.44%), the Pb BCA decreased (1763, 1317, and 1095 mg/kg), and the Ca BCA increased (6801, 6955, and 9368 mg/kg), which was attributed to PbCO₃ and PbSO₄ precipitation and competitive Ca and Pb absorption. High Ca²⁺ concentrations increased the strength of cation exchange, alleviated the fracture degree of fibrous roots, reduced the atrophy of vascular bundles, protected the cell wall, promoted C−O combined with Pb, enhanced the strength of O‒H, SO₄²⁻, C=O, and reduced the oxidization of alkynyl acetylene bonds.