Abstract: P-reasoning(packet reasoning) is obtained by the structure and dynamic characteristic of P-sets. P-reasoning is composed of internal P-reasoning(internal packet reasoning) and outer P-rcasoning(outer packet reasoning)，which has the dynamic characteristic and "sequence" characteristic. Backward P-reasoning (backward packet reasoning) is presented by the "opposite sequence" study for P-reasoning. Backward P-reasoning is composed of backward internal Preasoning and backward outer P-reasoning. hhe characteristic of backward P-reasoning is to find the attribute set changed in the reasoning conclusion using the change of the clement set in the reasoning condition. Using backward Preasoning, the paper gave P-attribute class, the characteristic and discovery-identification of P-attribute class characteristic, the application of P-attribute class in the information system.

Key words: P-reasoning, Backward P-reasoning, P-attribute class, Attribute class discovery theorem, Application