The exponential function is used to fit the curves between the beam parameters in the diffractive field of LP01 mode fiber end surface and the basic parameters of fiber. The fitting function between mode field radius in the near field, angular radius of far field divergence, beam propagation factor and the normalized frequency with the core layer radius are obtained respectively. The computed results from the fitting functions are very close to that from the analytical functions, when 1.5 ＜V＜ 9.4, the maximal relative error is smaller than 2%. In a certain degree, these conclusions will provide academic foundation for analyzing or calculating the beam parameters approximately.