Abstract: According to operational principle of pyrolysis gas chromatography and its judgment on water flooded formation, the new technique of judging water flooded formation applied pyrolysis gas chromatography data, combined logging data, well testing data and test production data, and carried on evaluation on water flooded degree by using the example of Fuyu oil field closed core well. Comparing the findings in well testing and test production, this method could improve accuracy of high questioned logging interpretation. However this method can't be applied to carbonate fractured reservoir, and it has better water flooded interpretation on condensate oil, light oil and middle weight oil and it has deviation of heavy oil and viscous oil comparing with the test production result. By using this method, the sample is affected a lot by time. pyrolysis gas chromatography combining fluorescence image could provide more accurate water flooded degree than logging interpretation.