Abstract: This paper presents a hybrid technique based on the time domain ray tracing (TD-Ray) and finite domain time domain(FDTD) methods for analyzing mixed-scale problems of predicting the scattered fields of IR-UWB through-the-wall signal. According to the ultra-short characteristics of IR-UWB signal, the TD-Ray method is used to analyze electrically large homogeneous walls and wide indoor area. It avoids the meshing process of FDTD method in a large free space area and increases the calculation efficiency while reduces the memory requirements. The FDTD method is applied to investigate area close to complex small-scale lossy structures, where the TD-Ray method suffers from the difficulty of large numerical errors. An example of applying the hybrid approach is given, to verify the validity of the model when both large-scale and small-scale exists. Then, the RCS for human being behind the wall is solved by the hybrid method, and the numeric results agree well with the measurements.