Revealing the main contributors to septic odor is a challenge since diverse compounds are responsible for this odor, and there might exist synergistic effects among different odorants. In this study, based on a reconstitution evaluation, the contribution of eighteen odorants identified in Huangpu River source water to the septic odor was explored. The reconstitution test result showed that the three typical odorants, including bis(2‑chloroisopropyl) ether (BCIE), diethyl disulfide (DEDS) and dimethyl disulfide (DMDS), with respective average odor activity values (OAVs) of 2.35, 1.65 and 0.78, only contributed 61 ± 3% of the FPA intensity for septic odor in Huangpu source water. At a BCIE concentration of 50 ng/L, synergistic effect for the septic odor occurred at 10 ng/L for geosmin and 20 ng/L for 2‑methylisoborneol (MIB), showing that coexisting musty odorants could enhance the septic odor intensity caused by some typical odorants. When both geosmin and MIB, with an average OAV of 4.54 and 1.38, were further included in addition to the three typical odorants, 88 ± 4% of the septic odor in Huangpu River source water could be explained. With addition of the remaining odorants with much lower OAVs (<0.23), 94 ± 2% of the septic odor could be explained. The musty odor was not affected by the presence of the co-existing odorants at the concentration levels of the present study. This study indicated that the overall odor of water contaminated with musty and septic odorants is a combination of both concentrations of individual contaminants and their synergistic effects. This is the first study to reveal the synergistic effects of typical musty odorants on septic odor, and the results of this study demonstrated that the synergistic effects of other odorants should be considered when dealing with the septic odor in drinking water.