A system level framework has been developed to simulate the UWB three-dimensional through-wall radar system and accurately predict its performance limitations. Only through a novel simulation model where Agilent ADS Ptolemy simulation is used combined with its Matlab co-simulation, is it possible to accurately simulate the UWB radar system. The developed simulation model has been validated experimentally using our UWB radar prototype. The system model can be easily extended to other UWB radar systems by simply changing the input pulse shape, UWB channel environment, wall characteristics, transceiver topology, etc. Various effects such as signal quality, pulse shape, wall characteristics can be easily investigated and re-optimized for high performance 3-D imaging using our developed model.