Abstract: We developed a fast resistivity imaging method for central loop transient electromagnetic sounding(TEM).Firstly,diffusion velocities were derived on the basis of propagation features of transient electromagnetic filed in full conductive space and half conductive space.Secondly,the all time apparent resistivity curve was calculated with numerical method after studying the half space function of vertical component of magnetic field.And finally,imaging formulas were given with diffusion velocity and all time apparent resistivity.The model tests and field example testified the validity of imaging for geo-electrical structure and electromagnetic sounding(MT)correction.