In the unified S-band （USB） tracking telemetering and control （TT&C） system，the ground station extracts the range information through the phase delay of the ranging tone transmitted by the satellite TT&C transponder. Considering the influence of the ranging tone extraction filter and channel noise on the downlink modulation system， the modulation index of the downlink ranging tone will fluctuate unexpectedly. So the system efficiency and communication distance of the whole TT&C system are affected. This paper introduces a ranging sound power decoupling method for a miniaturized USB TT&C transponder， which simplifies the design of the uplink receiver while eliminating the adverse effect of the uplink received signal strength on the modulation system through the normalisation of the received power， so as to guarantee the power efficiency of the modulation signal of the measurement and control transponders. The feasibility and effect of the method are verified by product measurement.