A numerical calculating method,which has been simplified to a one dimension integrating formula,has been used to improve the accuracy in evaluating the coupling factor and receiving power of the optical fiber.The concordance of the results of numerical calculating and measurements for the 62.5/125 multimode optical fiber show that the Gaussian beam could describe the light intensity distribution of optical fiber well.The accurate numerical method for calculating the coupling factor and the receiving power at near field of the optical fiber is necessary to diminish the error risen from some approximate methods published.