This paper firstly discusses some problem about the uncertain reasoning machine in the agricultural expert system based on HPC, such as the knowledge representation, the certainty factor, the representation of evidence uncertainty, the calculation of multi evidence uncertainty, the update of uncertainty and so on. Secondly, the control strategy of this uncertain reasoning machine is discussed. This control strategy adopts a converse reasoning strategy based on target, and it adopts a conflict removing strategy based on certainty factor. The arithmetic to realize the uncertain reasoning machine is also given in detail. At last, this paper stats the example of malnutrition diagnosing of grape as the application of the reasoning machine.