Residential Thermostatically Controlled Load (TCL) is an effective resource to participate in power grid active power balancing. However, its distributive characteristic makes control center difficult to directly get its aggregated power. This paper proposed a convenient method to estimate the aggregated power of residential TCL load, which can easily help to confirm its approximate value and upper and lower limits. In advance, this paper introduced TCL into system frequency regulation. The result of simulation proved that TCL is conducive to maintain the system frequency, which proves the effectiveness of the control strategies.