With hybrid method of BPM and FSRM , the power of transmission was calculated in the case of SOI Rib Waveguide to Fiber coupling .A simple integral formula was developed to work out the reflectivity. Based on the results acquired with the hybrid method, normal FSRM methods and Fresnel equation overlap integral method ,it was deduced that Fresnel equation method and overlap integral method can be used together only when the two waveguides on the each side of disconnection facet are weakly guided.