Abstract: In order to carry out equivalent study between defect risks and failure defects,the event tree analysis(ETA) method was introduced to determine the risk flow route of automobile defects.An SPN model for risk assessment was set up based on characteristics of automobile defect risks,and a 3D matrix graph was introduced to describe overall risks.According to the scattered and fluctuant characteristics of automobile defect data,a risk forecast method based on the gray theory was proposed,and a failure forecasting model of automobile defects was built with failure data as the basis of forecasting risk possibility. The precision of the failure forecasting model was proved by residual discrimination. It is shown that on gathering actual failure data from after-sales service, the gray model has a favorable applicability for forecasting risk possibility.