During the operation of civil aircraft, occurrences which affect the operation safety may exist due to design, manufacturing defects, the exceeding operation and environment condition. Presents the sources and classification of occurrence by the definition in the continued airworthiness. The risk assessment processes have been established based on the risk management theory. The severity and probability parameter of occurrence are defined and calculated by the developed method in this paper. Then the risk level of occurrence is obtained. The quantitative risk assessment of aircraft and fleet is also given to assess occurrence effect when the occurrence risk under unacceptable level.