Eggshell is a cheap and environmentally friendly calcium source. In this study, Ca-modified biochar adsorbents (CEA) were prepared by 1:10, 1:2, and 1:1 mass ratio of the eggshell and Eupatorium adenophorum. The CEA-2 sample prepared with a 1:2 mass ratio showed a maximum Pb adsorption capacity (97.74 mg·g^-1) at the conditions of an initial pH of 7.0, an adsorbent dosage of 0.5 g·L^-1, and a contact time of 8.0 h. The kinetic and isotherm studies indicated that the adsorption process of the CEA-2 sample had monolayer adsorption characteristics, which was controlled together by intraparticle and interface diffusion. Thermodynamic studies indicated that the adsorption process of CEA-2 was spontaneous (ΔG ⁰ 0). X-ray diffraction and scanning electron microscopy analyses showed a uniform distribution of Ca-Pb precipitation on the CEA-2 surface, which proved that chemical precipitation was the main adsorption mechanism. Fourier transform infrared spectra found that CEA-2 had abundant active groups, especially nitrogen-containing functional groups, which could adsorb Pb through a surface complexation reaction. The Brunauer-Emmett-Teller surface area of CEA-2 was found to be 621 m²·g^-1, and such developed pores could ensure the smooth diffusion of Pb. Finally, the effect of coexisting cation and anion experiment and the cyclic regeneration experiment indicated that CEA-2 had prominent stability and reusability for Pb adsorption.