Abstract: Cooling tower hydraulic turbines, as an important energy saving device in recycling the residual pressure of industrial circulating water system, have been widely used in practice for driving the fan in replacement of motors. Such a turbine-fan unit, with the turbine and fan in series connection, often works at an ultra-low specific speed in a pressured environment, which differs from the conventional power turbines. This paper presents a theoretical study on the working characteristics of a hydraulic turbine in a turbine-fan unit. First, we found that the fan speed is proportional to the turbine flow, namely it is self-adaptive in characteristics ? the speed of the turbine-fan unit is proportional to the flowrate of cooling water, producing the right air flow exactly demanded by cooling water. Second, in the condition of system pressure and flow always changing, the unit speed and unit flow of the turbine are constant, or the turbine always maintains an operating condition of self-similar characteristics. This means that the turbine-fan unit can maintain an optimal working condition if its design condition is well adjusted, different from the power turbine that needs a complicated control on its working conditions. Finally, all these characteristics of working conditions are validated by our experimental results. Thus, such a turbine-fan unit does not need an adjusting system, greatly reducing its construction and operating costs, and the characteristics revealed in this study would be very useful to the design and operation of residual pressure recycling turbines.