Abstract: This paper is to present our research results of degradability of quaternary ammonium salt(QAS) in nature seawater. For our research purpose, we have conducted three experiments to study on the effect of bacteria contents, light source and temperature on the degradation rate of QAS, including biodegradation, photolysis and hydrolysis experiment. The results of our research indicate that the biodegradation curve of QAS is divided three phases along with the quantity of bacteria. The biodegradation rate is slower in the middle stages than other two stages because of the killing or inhibition effect of QAS on bacteria. The biodegradation rate is 8.93% at 192 h. Light source and temperature could affect greatly the photolysis and hydrolysis rate of QAS. kuv/kvis is 188 and k40℃/k20℃ is 4.58. The degradation of QAS is mainly composed of photolysis which reaction rate constants is 1.96 and 4.5 times of biodegradation and hydrolysis respectively at 20℃. Three types of degradation of QAS could be well fit to a first-order kinetic model, which degradation reaction rate constants are 0.00046, 0.0009 and 0.0002 respectively and half-life is 62.8, 32.1 and 126.0 days respectively at 20℃. The long half-life of QAS states that it is necessary to pay attention to residual persistence of QAS in marine environment.