The power confinement factor of the fiber with cladding made of uniaxial crystal material whose optical axis is parallel to the axis of fiber was investigated by means of numeric solution and the influence of k_cl on the reflectivity of this kind of fiber Bragg gratings was physically interpreted with mode characteristics.The calculated results indicate that for HE₁₁ mode,more power is transmitted by the core of the fiber with cladding made of isotropic material than that with cladding made of uniaxial crystal material and the degree is dependent on the parameter k_cl,the reflectivity of the fiber Bragg grating with cladding made of uniaxial crystal material is much higher than that with cladding made of isotropic material,the reflectivity of this kind of fiber Bragg grating is greatly influenced by the energy distribution between the core and the cladding of this kind of the fiber.