The observations of very-high-energy (VHE, E > 100 GeV) gamma rays provide an important probe to study the gamma-ray sources, gamma-ray bursts, and AGN. Ground-based extensive-air-shower (EAS) array of charged particle detectors plays a key role in VHE gamma rays due to the large active area, the high duty cycle, and wide field of view. However, the effective gamma-ray detection threshold of existing EAS experiments is limited to 500 GeV $\sim$ 1 TeV because of the low detection efficiency and poor gamma/hadron separation in the energy region below 1 TeV.
In order to extend the detection threshold to 100 GeV, the site must be at a higher altitude, at which the maximum EAS shower caused by gamma rays below 1TeV will be generated.
In this paper, we focus on discussing how the observation sensitivity of low energy gamma rays will improve as the altitude of the observation site increases.
We have carried out a detailed Monte Carlo simulation to study the performance of the charged particle detector array for cosmic rays at high altitudes from 4300 m to 5700 m. As the main types of particle detector, the Water Cherenkov Detector array (WCD) have been arranged in the Geant4 simulation, and three altitudes of 4300 m, 4900 m and 5700 m have been set in CORSIKA. The detection efficiency, the gamma/hadron separation, the angular resolution, and the energy resolution will be discussed in this work