Biochars have been modified by alkali (Ca(OH)2) to enhance Cd sorption capacity in aqueous solution. In this research, the alkali-modified (Ca) biochars were prepared by co-pyrolyzing lime (Ca(OH)2) and soybean straw (SBB) or rape straw (RSB) at 450 °C. The absorption mechanism was investigated by a series of experiments and was provided by quantitative analysis. The maximum adsorption capacities of Cd2+ by Ca-SBB and Ca-RSB were calculated to be 78.49 mg g−1 and 49.96 mg g−1, which were 1.56 and 1.48 times higher than SBB (50.40 mg g−1) and RSB (33.79 mg g−1), respectively. Compared with the original biochar (SBB, RSB), alkali-modified biochars (Ca-SBB and Ca-RSB) were found to have faster adsorption kinetics and lower desorption efficiencies. The mechanism study indicated that Ca(OH)2 modification effectively enhanced the contribution of ion exchange and decreased the contribution of functional groups complexation. After Ca(OH)2 modification, precipitation and ion exchange mechanisms dominated Cd2 + absorption on Ca-SBB, accounting for 49.85% and 34.94% of the total adsorption, respectively. Similarily ion exchange and precipitation were the main adsorption mechanism on Ca-RSB, accounting however for 61.91% and 18.47% of total adsorption, respectively. These results suggested that alkali-modified biochars have great potential in adsorbing cadmium in wastewater.