Abstract: In order to realize the aim of rapid, efficient and large-depth exploration, we utilize the in-loop device with large rectangular transmitter in transient electromagnetic exploration.That is, the TEM (transient electromagnetic method) detects the target in some range of the loop.However, the response equation of the non-central point in the loop is complex and does not have the analytical resolutions, leading to the difficulty to get the apparent resistivity.In most circumstances, we use the calculation equation of in-loop to calculate the non-central point, leading to the decrease of the interpretation accuracy.This paper gives the response equation of the circular loop and separates the transmitting loop into electric dipoles.Then, we discuss the superposition response from the dipole and equivalence between the circular loop and inscribed polygon, calculate the response value of any point in the loop and analyze the distribution characteristic of large-loop based on the electric dipole approximation.At last, we realize the conclusion that observation at the non-central point in the large-loop is feasible, which has the theory and application importance.