Abstract: As for transient electromagnetic method(TEM), turn-off time got so much concern that the importance of electrifying time (excited-field build-up time)for transmit-loop is always neglected. Normally,setting certain sampling frequency which has enough observation time window, same build-up time is set. This thesis has been studying about building asymmetry “transmitting-observing” time window array, for improving accuracy of TEM. As for positive or negative polar observation time window, observation time window or build-up time was a quarter of period. This thesis mainly proceeds with Fourier transform of electric signal, then frequency spectrum of different build-up time was calculated accurately. Influence of build-up time to frequency spectrum was concluded by theoretical derivation and numerical calculation. The results suggest that high frequency components lost much with build-up time getting more, then shallow information was influenced, inversion depth of high frequency signal should not be more than shallow layer shadow depth definitely. Meanwhile, low frequency components gather much with build-up time getting shorter. limited by detection depth, shorter build-up time was unnecessary. Eventually, by quantized calculation, reasonable build-up time interval was concluded, that is build-up time of TEM should be limited between 61.0 μs-12.7 ms, and asymmetric building-observation time window between 1∶1 from 1∶8 were built.Then, more appropriate time window was built for detecting different depth-no deeper than 800 m.