To investigate the influence of Magneto Optical(MO) properties of a prism on systematic optical response, the MO spectra of a single BK7 prism under the condition of total internal reflection was measured, and the spectra of Kretschmann configuration composed of the BK7 prism and a gold film was measured also. Then, based on the framework of traditional optical theory, the reasons of these spectra were analyzed explicitly. The theoretical results show that under conditions of total internal reflection, the Kerr rotation of the single BK7 prism increases significantly before the system gets in the critical angle, and the phenomenon comes from the different reflection coefficients between p and s waves. When the BK7 prism with the gold film is combined to form a standard Kretschmann configuration, a huge enhancement of Kerr rotation appeares at the resonance angle due to the MO capability of BK7 material and the conversion of incident p-polarized wave to surface plasmons. The maximal experimental polar Kerr rotation of the gold film (26 nm) is 1.7°, which overcomes the disadvantage of poor response in the traditional way.