The all-time apparent resistivity iteration inverse method was deduced in detail based on the current slanting wave of the TEM central loops unit. In the forward process, the turn-off time integral was represented analytically. The resistivity derivation could be transformed to the time derivation through the variable transformation in the inverse process. The calculation precision and speed were promoted by avoiding the numerical integral and differential calculation used in approximating the derivation. The standard database method was used to decrease the calculation time of probability integral in the inverse process of iteration as well as to improve the precision and speed. Then the survey data were processed and some problems were discussed on the basis of a few calculation examples.