Abstract: Back-projection imaging algorithm is widely used in through-the-wall and ground penetrating radar(GPR) imaging because of the high image quality and being compensated easily. This paper introduces the Back-Projection(BP) algorithm to the stepped-frequency through-the-wall radar and proposes the time-minimization method to eliminate the wall's effects, such as refraction, changing in speed and attenuation. Proof of concept is provided using real data collected in a laboratory environment. The results show that the BP algorithm outputs a high quality image and the time-minimization method can eliminate the wall's effect very well.