NOVELTY - Increasing salt-tolerant ability of fresh water activated sludge nitrobacteria comprises using salinity detection probe to real time measure salinity in water, sending signals to automatic control system upper computer via salinity detector, using the automatic control system upper computer to judge the setting value beyond or not and sending relevant control command to control a metering pump, and using the metering pump to ceaselessly extract potassium concentrated solution in potassium concentrated solution storage tank to add into aeration tank according to the control command.    USE - A method for increasing salt-tolerant ability of fresh water activated sludge nitrobacteria.    ADVANTAGE - The method is simple, has easy engineering operation and realizes stable and efficient biological treatment effect under the condition of not diluting salinity and desalting.    DETAILED DESCRIPTION - Increasing salt-tolerant ability of fresh water activated sludge nitrobacteria, which is based on a hardware system composed of a salinity detection probe, a salinity detector, an automatic control system upper computer, a concentrated solution storage tank and a metering pump, comprises:    (A) preparing potassium chloride in a potassium concentrated solution storage tank into potassium chloride solution with certain concentration C (mg/L), using an impeller agitator to ceaselessly stir to keep the concentration stable;    (B) making water outlet pipes of the salinity detection probe and the metering pump in water inlet end of the aeration tank of sewage treatment plant, guaranteeing under the liquid level;    (C) using the salinity detection probe to real time detect the salinity in water, sending signals to the automatic control system upper computer via the salinity detector;    (D) using the automatic control system upper computer to judge the setting value beyond or not according to the signals, processing the step (E) if the salinity is beyond the setting value, or processing the step (F);    (E) using the automatic control system upper computer, controlling the flow speed Q2 of the metering pump according to formula: Q2=50Q1/C, where Q1 presents the inflow flow of the aeration tank, if the automatic control system upper computer judges the salinity beyond the setting value, using the metering pump to ceaselessly extract the potassium concentrated solution in the potassium concentrated solution storage tank to add into the aeration tank according to the flow speed Q2; and    (F) closing the metering pump when the automatic control system upper computer judges the salinity below the setting value.