In order to solve the problem that traditional SPR theory is approximate when determining parameters of metal film based on Kretschmann geometry, the strict thin film optical theory is introduced. SPR attenuation curve is obtained with characteristic matrix of film system. The results show that there exists differences in resonance angle and amplitude of reflectivity between approximate SPR theory and thin film optical theory. Contour map of resonance angle deviation shows variation of resonance angle deviation under different dielectric constants of metal film. Further experimental study shows that the simulation result of thin film optical theory is more consistent with experimental data than of approximate SPR theory. Finally the sensitivity of SPR sensor is discussed using approximate SPR theory and thin film optical theory respectively, and as a result, there is great difference in distribution region of sensitivity. Optical parameters of metal film can be determined more exactly using thin film optical theory, and accurate parameter combinations must be benefit to design high sensitivity SPR sensors.