Abstract: This paper introduces a method of calculating original oil & gas saturation using capillary pressure data. On this base, deep analysis has been made on its uncertainty from many aspects. Influence of uncertainty on reservoir evaluation in saturation calculation has been opened out. So as to the influence of reserves, production project and engineering project has been opened out. It becomes the risk and opportunity ultimately in projection. So the importance of uncertainty analysis in oil & gas field exploration and production has been opened out.