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Focal length of multi-core image fiber is a crucial parameter for the performance of fiberscope. In this letter, we present a novel method that includes simulating the transmission process of waveguides by inputting a plane wave in the wavelength of 0.5 mum in cores using finite-difference time-domain (FDTD) method, integrating the power outputs in the end of the core and conforming the numerical aperture (NA) corresponds to the descending area of the output power curve. According to the geometry structure of the fiber, we can calculate the focal length of image fiber conveniently. Computer simulations of a multi-core image fiber with 2.20 mum core diameter show that the focal length is 1.258 mum. The numerical experiments have demonstrated that the FDTD method can be used to calculate the focal length of image fiber accurately.