First of all, a goal-guiding graphic reasoning approach that based on the predicate/transition system has been proposed for the first-order predicate logic. In process of reasoning, the premise is separated from the conclusion, which has been taken as the beginning of the backward reasoning that is purposeful and effective as well. Next, this reasoning approach has been applied in the agriculture expert system to present a method of solving problem, providing a new way for studying the reasoning mechanism of the agriculture expert system.