This paper studied the phtodegradation behavior, mechanism and security of propranolol (PRO) in water under ultraviolet irradiation using the high pressure mercury lamp. The photodegradation mechanism was verified by reactive oxygen species (ROS) quenching method, and the photoproducts security was evaluated by luminescent bacteria toxicity test. The results showed that the rate of photolysis (k) of PRO decreased with increasing initial concentration, and showed a significant negative correlation (r2 > 0.95). The increasing k value was also accompanied with higher pH of the solution, and the k values were 0.0953-0.267 min(-1) under pH 5-9. ROS quenching experiments showed that the PRO UV photolysis process included a triplet PRO (3PRO*) direct photolysis participation, and self-sensitized photolysis participation caused by hydroxyl radical (·OH) and singlet oxygen (1O2). Direct photolysis rate was greater than the rate of self-sensitized photolysis. The result of 1O2 steady concentration measured by FFA as the probe was consistent with the quenching method. The toxicity evaluation illustrated the formation of some intermediate photoproducts, which were more toxic than PRO.