  We estimate the sensitivity of LHAASO telescope for the large angular scale
diffuse gamma-ray flux in multi-TeV - multi-PeV energy range. We discuss
possible sources of the signal in this energy range including the guaranteed
flux from cosmic ray interactions in the interstellar medium and possible flux
from decaying dark matter. We show that LHAASO will be able to detect the
diffuse cosmic ray induced gamma-ray flux up to high Galactic latitude regions
thus providing firm identification of the Galactic cosmic ray component of the
astrophysical neutrino signal detected by IceCube and clarification of the
nature of the knee feature of the cosmic ray spectrum. Comparing the diffuse
flux sensitivity with the diffuse gamma-ray flux expected from the dark matter
decays, we show LHAASO will be able to detect the gamma-ray signal from dark
matter particles of PeV-EeV mass decaying on the time scale up to 3e29 s.
