The abuse of antibiotics has led to a large amount of antibiotics residues in the environment, which seriously threatens the health of offshore ecosystems and human safety. Sludge biochar can effectively inhibit the proliferation and diffusion of antibiotics in the offshore environment through adsorption and degradation mechanisms. Among them, the resource development and utilization of ceramsite modified sludge biochar (CMSB) provides a new idea for the control of antibiotics pollution in the offshore environment. However, the adsorption effect and mechanism of CMSB on antibiotics in seawater are still unclear. Therefore, in this study, the adsorption characteristics of CMSB with different clay (kaolin, attapulgite) contents were investigated and compared under different initial antibiotics (sulfamethoxazole and tetracycline) concentrations. The results showed that the adsorption capacities of CMSB1, CMSB2 and CMSB3 for tetracycline solution were 1.06 mg g−1, 1.36 mg g−1 and 1.39 mg g−1, respectively, at a concentration of 5 mg L−1. Compared with CSB (1.25 mg g−1), CMSB3 increased the most, which was 11.2%. At the concentration of 10 mg L−1, the adsorption capacity of CMSB1, CMSB2 and CMSB3 to tetracycline was significantly enhanced than the concentration of 5 mg L−1, which was 2.27 mg g−1, 2.49 mg g−1 and 2.71 mg g−1. The adsorption capacity of CMSB to sulfamethoxazole showed a similar trend. This study provides experimental support and theoretical basis for environmental and economic remediation of antibiotic pollution in coastal seawater.