As the TOA ranging error was always corrupted by the direct path blockage and excess delay in complicated indoor environments,the NLOS ranging error due to IR-UWB through wall propagation was modeled by dividing it into two parts: the geometrical error caused by the pulse transmitted through wall,and the peak biased error caused by the pulse waveform distortion due to frequency dependence of huge bandwidth.The theoretical geometrical error bound was derived from transceiver distance,wall thickness and materials.The IR-UWB through wall propagation was modeled using sub-band method,and the peak biased errors were investigated through the waveform distortion of IR-UWB signal propagated in different wall structures.The simulation results show that the geometrical error was mainly determined by the obstacles parameters and the peak biased errors caused by IR-UWB waveform distortions were within the distance resolution of IR-UWB signal.