Galactic diffuse gamma ray emission (GDE) is introduced by the galactic cosmic rays (CR) interacting with the interstellar medium (ISM) and radiation fields (ISRF). The GDE is a very important probe of CR propagation and interaction. Different from the measurements of CR particles in the local vicinity, the GDE enables a direct measurement of CR distribution in the Milky Way, and can thus provide much more important information of the production and propagation of CRs. LHAASO-WCDA, with an large area of 78000 $m^2$, has an excellent ability to study VHE gamma-ray astronomy and GDE. In this proceeding, method of background estimation and some techniques to reduce the contaminant of resolved gamma-ray sources are briefly introduced. At last, diffuse emissions from the inner and outer Galactic plane are detected with $27.9\sigma$ and $11.9\sigma$ significance, respectively.