Sulfonamide antibiotics in the environment not only disrupt the ecological balance but can also enter the human or animal body in various forms and cause harm. Therefore, exploring efficient methods to degrade sulfonamide antibiotics is crucial. In this study, we prepared biochar (BC) using corn straw, and TiO₂/BC was obtained by doping different proportions of TiO₂ into biochar with varying carbonization temperatures using the sol–gel method. Next, we investigated the degradation of sulfamethoxazole (SMX) in solution using the generated TiO₂/BC under ultraviolet irradiation and studied the effects of various experimental parameters, such as the type of composite material, composite material addition, solution pH, and initial antibiotic concentration on SMX degradation. Under an initial SMX concentration of 30 mg/L, the composite with the best photocatalytic degradation performance was TiO₂/BC-5-300 (i.e., 5 mL of TiO₂ doping; 300 °C calcination temperature), with an addition amount of 0.02 g and a solution pH of 3. The degradation efficiency increased from 22.3% to 89%, and the most significant degradation effect occurred during the initial stage of photocatalytic degradation. In the TiO₂/BC-5-300 treated SMX solution, the average rhizome length of bean sprouts was significantly higher than that of the untreated SMX solution and slightly lower than that of the deionized aqueous solution (3.05 cm < 3.85 cm < 4.05 cm). This confirmed that the photocatalytic degradation of SMX by the composite was effective and could efficiently reduce its impact on the growth of bean sprouts. This study provides essential data and theoretical support for using TiO₂/BC in the treatment of antibiotic-contaminated wastewater.