Abstract: A three dimensional imaging algorithm applied to the through wall radar is presented in this paper. A math ematic model of through wall electromagnetic wave is constructed. The path of microwave penetrating wall is resolved. The time delay is compensated according to microwave passing through wall. An improved back projection method of through wall imaging based on the radiation model of dipole is deduced. The algorithm flow chart is also given. With the sampling data from FDTD, the three dimensional imaging results are obtained according to the algorithm flow chart. The influence of the wall and the coupling between targets are analyzed. The simulation results show the efficiency of the algorithm.