Abstract: The comets are consisted mainly of water and dust and around cometary nuclear there are ionized gases mainly made up of water and hydrogen. Dust grains in comet will become charged. As a new composition, charged dust grains will take part in cometary plasma group behaviors. Three main charging mechanisms are considered in this paper. First, charged plasma particles spray onto surfaces of dust grains will be collected, this current is presented by J₁; second, photoemission caused by the solar rays, the photoelectron current is J₂; third, J₃, current of secondary electron eject. At a certain stage, dust grains will stop charging and attain an equilibrium-state, i.e., ∑ J_α = 0, α = 1,2,3. Considering that comets are plasma environments of H₂O⁺, and the main compositions of cometary dust are silicate and carbon, based upon observational data of comet Halley and comet G-Z, the charging characteristics and equilibrium potentials of dust grains are calculated. A curve of equilibrium potential at different distance from the nuclear of P/Halley is provided. It is found that the equilibrium potential is obviously correlative with electron density. Using algebraic equations about equilibrium potential and logarithm of electron density, too, the calculations showed that the equilibrium potential has little relativity with other plasma parameters such as electron temperature. Generally, these conclusions can be suitable for other plasma comets.