Mathematical statistical method (MSM) is a very important wastewater treatment plant data processing analysis tool. In this study, MSM had been used to evaluate the efficiency, stability and reliability of the low-strengthen complex wastewater treatment system. Results showed that while LSCWWTs upgrading in the future, each influent parameter could be set as the follows: COD = 738 mg/L, BOD = 300 mg/L, SS = 454 mg/L, TN = 64.3 mg/L, NH3-N = 65.2 mg/L and TP = 7.65 mg/L. For effluent of LSCWWTs, the stabilities of COD, BOD, SS and TN were all A, TP was B, and NH3-N was D. For effluent COD, TN and NH3-N reached the first class discharge standard of A, the reliability probabilities were 83.89%, 80.23% and 99.43%, respectively. And the reliability probabilities of effluent WQPs reached the first class discharge standard of B were all more than 98%.