Imitated the optical system of Rowland circle structure, the Rowland sphere structure is built in this paper, the Rayleigh-Sommerfeld diffraction scalar integral formula of electromagnetic wave is simplified on the Rowland sphere structure, the computational formulas of spatial frequency spectrum and diffractional far field distribution from the weakly guiding optical fiber LP mode are derived. The concept of central disc in LP 01 mode optical fiber diffractional far field, which had analogy to the Airy disc of plane wave Fraunhofer diffraction from circular aperture, is presented, and the calculational formulas of LP 01 mode optical fiber diffractional far field divergence half width is deduced in the end of this paper.