The effective sewage intake technology is prerequisite for the untreated sewage source heat pump system to run steadily. In terms of the insufficiency of current sewage intake technologies, an open sewage intake integration technology is proposed. The mathematical model and the distinguish basis of critical state are established for the sewage intake characteristic by defining some important operating parameters, such as the blocking coefficient, the contamination density and the section coefficient. Furthermore, the change rule of each critical operation parameter is observed along with the various initial parameters. The result can provide the theoretical basis to design the open intake integration technology.