In this paper, a LiNbO3 optical-fiber modulator with CPW electrode is discussed. Being use the LiNbO3 optical fiber as the transmission channel of light in this structure, the modulator has less LiNbO3 than the conventional LiNbO3 waveguide modulator. It effectively decreases the effective dielectric coefficient of microwave and realizes velocity matching and low half-wave driving voltage. The characteristic impedance, refractive index and bandwidth of the modulator have been derived using finite element method. The optimal structure of modulator is presented.