Hybrid diagnostic model (HDM) is an extension of diagnostic dependency model, it is a model which makes function and failure mode model into one system model. This paper studied the hybrid diagnostic inference rules in the model, and proposed the hybrid diagnosis reasoning based on the failure mode and function, and the specific divided into five different inference rules. The modeling of hybrid diagnosis model reasoning process has carried on the detailed description, maked the relationship between the failure rate function and fault patterns clear. The experiments showed that a hybrid diagnostic model is a kind of effective model is suitable for fault analysis, applying the model to our equipment fault analysis is feasible and effective.