Abstract: A five-compartment purification tank reactor (FcPTR) with the same volume as three-compartment septic tank (TcST) was constructed to address the problem of substandard effluent from TcST, and a study of the start-up characteristics of FcPTR and TcST for treating actual domestic wastewater was conducted. The results showed that the removal rates of FcPTR and TcST of COD_Cr decreased to 44% and 31%, respectively, on the 13th day (HRT is 80 h). On the 46th day (HRT is 36 h), the removal rates of FcPTR and TcST of COD_Cr increased to 86% and 61%, respectively, by reducing HRT. On the 46th day, pH of FcPTR anaerobic fold 1 zone was generally larger than that of anaerobic fold 2 zone, and the acid to basic ratio of FcPTR anaerobic zone was 0.12-0.20, which had better acidification and methanation separation characteristics. On the 46th day, the filler and sludge in FcPTR were observed by sweeping electron microscopy, the morphology of biofilm on the fillers was successful, and the microorganisms in the sludge were mainly short bacilli and spherical bacteria. Research shows FcPTR had a better treatment effect than TcST, and COD_Cr effluent reached the Class Ⅰ A of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants (GB 18918-2002).