Abstract: Due to the fact that the geological structures are complex and the geological feature parameters are hard to predict, the drilling operation in complex formations is characterized by high risk and high investment. Therefore, reasonable discrimination of downhole engineering risk degrees is of great significance to risk decision for drilling engineering program. For this problem, a model is built for discriminating the risk degree based on risk probability and fuzzy theory. Firstly, calculate the risk probability of common downhole engineering risks. Then, based on which and in conjunction with the incident records in the well history data use fuzzy mathematical theory to construct the subordinating degree function between various degree intervals. Finally, delineate the intervals according to subordinating degree, hence realizing the discrimination of engineering risk degrees. After outlining its discriminating principle, then the drilling data from somewhere in the west is taken as an example. Example analysis shows that the discriminating result of this method almost agrees with the actual engineering and satisfies the real need of drilling engineering and can provide basis for risk discrimination for the design of drilling engineering program.