The presence of emerging contaminants in the raw water puts forward new challenges to drinking water treatment process. The chlorination of typical sulfonamides antibiotic sulfadimethoxine (SDM) in the sodium hypochlorite disinfection process was studied. The chlorination effects, including the initial concentration of residual chlorine, the pH value of the solution, ammonia concentration, and the inorganic anions, were also investigated. The mechanism of SDM chlorination was explored and the ecological risk was further assessed. The results showed that, under the conditions of the initial SDM concentration 15 μmol.L^-1, the initial concentration of residual chlorine 60 μmol.L^-1, the SDM removal reached high of 95.9% in 120 s. The degradation process conformed to the pseudo-second reaction kinetics. The degradation rate constants decreased with the increasing ammonia concentration, and increased with the initial concentration of residual chlorine. Background anions Cl^-, NO3^-, SO4^2- had little effects on the reaction while HCO3^- and CO3^2- had Inhibition effects on the reaction. SDM chlorination can be improved under neutral conditions. Nine degradation intermediates are identified by using high-resolution mass spectrometry HRMS Orbitrap. The degradation process covers chlorination, demethylation and hydroxyl addition reactions. The complete removal of SDM does not imply an effective reduction of ecological risk during disinfection process which poses a potential risk to the safety of drinking water quality.