Abstract: To identify lost circulation formation is critical to successfully solve lost circulation problems. It is also a hard nut to crack in drilling complex formations. To prevent lost circulation and protect reservoirs, it is necessary to identify the lost circulation formation. The traditional methods are used to identify according to the properties of lost circulation after it occurs, which inevitably result in loss of drilling fluid and pollution of reservoirs. To solve the problem, based on the mechanical treatment and analysis of a lot of lost circulation parameters acquired from oilfields, using the theory of fuzzy mathematics, according to the relationship among lost circulation parameters, the relationship between lost circulation exent and Euclid approaching extent is determined, as well as a model used to identify lost circulation formation and predict pre-drilling risk is established. Field application shows that the model features in accuracy, good effetiveness, and low cost. The technology has good performance in predicting the risk of lost circulation formation.