The heat transfer mechanism of sewage in the pipes was analyzed. It was found that the heat transfer was associated with a variety of factors and the effects of the air above the sewage on its temperature varied with seasons. The all- day temperature of sewage in pipes of one residential quarter in Chongqing was measured by a thermocouple thermometer. And the reasons oftemperature variation were analyzed. The temperature variation of sewage in different sites was studied by using daily range temperature，standard deviation and etc. The reasons were analyzed as well. Based on the conclusion, the selection ofthe intake point ofsewage source heat pump systemwas discussed.