The surface potentials on fluffy dust grains consisting of carbon and silicate materials in cometary plasma environments are calculated based on continuous charging model including collection, secondary emission on dust surface. The equilibrium potential and dust fragmentation criterion are given out for dust grains at different distance from comet P/Halley and P/G-Z. It is suggested that the electrostatic disruption is one of the possible causes for large amounts of tiny dust grains in coma.