The electric field inside the cavity of a uniformly charged thick ellipsoidal shell is calculated. This shell is composed of non-conductor. Its inner and outer surfaces are two coaxial ellipsoidal surfaces with the same eccentricity ratios. The method is used to divide this thick ellipsoidal shell to infinite thin ones. Firstly，the charge distribution of the thin ellipsoidal shell is calculated. In comparsion with the charge distribution of the charged conducting ellipsoidal surface，the electric field in the thin ellipsoidal shell is found. Finally，with the help of the superposition principle of electric field，the electric field in the cavity of thick ellipsoidal shell equals zero is found.