Detection of Gamma ray source is one important approach for searching galactic cosmic ray origins, meanwhile the high energy diffuse gamma ray is expected to be produced by astrophysical objects, including dark matter annihilation and decay emissions in galactic or extragalactic structures. The large high altitude air shower observatory (LHAASO) is a hybrid detector array. It does not have only 1 km$^2$ detection area, but also powerful background discrimination. It has been the best observatory for the all sky survey for the gamma-ray sources and diffuse gamma ray above several TeV. we propose one characteristic variables to discriminate the gamma ray from hardon based on the feature of LHAASO. The lateral distribution of extensive air shower is divided into different sizes of rings, then the particle density is calculated. We extract characteristic variable ($\rho_{w200-220}/\rho_{k0-20}$) which is the ratio between density of the ring from 200 meters to 220 meters in the water Cherenkov detector array and that from 0 meter to 20 meters in the 1 kilometer squarer array. In this paper, we present preliminary results for the abilities of the variable.