One of the main physical goals of LHAASO is to accurately measure cosmic ray energy spectra
for individual or group mass compositions. LHAASO contains four types of detectors, which can
provide multiple mass sensitive observables, such as muon content, Xmax, etc. In this paper, we
will focus on the steepness of the lateral distribution of the air shower. The steepness is defined
as S = D_WCDA/D_KM2A , where D_WCDA is the photoelectron density of 200-220 meters from
the shower axis measured by WCDA and DKM2A is the particle density of 0-20 meters from the
shower axis measured by KM2A. Since the lateral distribution of the iron shower is flatter than
the lighter nuclei shower, the steepness is a powerful mass sensitive observable. Combined with
other mass sensitive observables, LHAASO can select iron nuclei with 90% purity.