A scheme for atom localization via measuring population of the upper level in a quasi-Λ-type four-level atomic system is proposed. The function of conditional position probability distribution based on population of the upper level is obtained by solving the Schr?dinger equation using the perturbation theory. The position and width of the atomic location peak are analyzed theoretically and their equations are obtained. It is found that when the coupling field and probing field are in accord with the configuration of electromagnetically induced transparency (EIT) or when the probability amplitudes of the atom transitions from the upper level to the near-degenerate lower levels are destructive interference, atomic location cannot be obtained. But near the EIT window, the atom can be localized precisely. The peak position and its width of atom location are determined by the coupling field intensity and the space between the near-degenerate lower levels.