Ultrasonic disinfection is safe and effective, but requires high power consumption. For economic considerations, ultrasound and chlorine can be combined to promote a wider range of applications. In this paper, we investigated the disinfection effect of secondary effluent with low-frequency ultrasound and sodium hypochlorite. The optimal conditions were determined by changing the ultrasound intensity, exposure time, chlorine dosage, and chlorine contact time. The results showed that ultrasound can significantly enhance the effects of chlorine disinfection. When the coliform bacteria is 106~107CFU/L, the dosage of chlorine at 4~6mg/l can meet the national discharge standard (less than 104 CFU /L). However, the chlorine dosage can be further reduced to 1~2mg/l if we add 30 s of ultrasonic radiation, thereby reducing the disinfection by-products. Orthogonal experimental results show that the relative importance of the disinfection factors: chlorine dosage> ultrasonic irradiation time >chlorine contact time> ultrasound intensity.