Agricultural commodities trading between leading enterprise and farmers is essentially a long-term cooperation relationship，the effective cooperation basis between the two parties is self-enforcing contract for long-term effective encouragement and default risk reduction. Based on the definition of stationary relational contract of agricultural products trading，under the situation of the existence of specific investment of farmers and leading enterprises having all ex-post negotiation capacity，the possible form of optimal stationary relational contract of agricultural products trading is analyzed，and the conclusion indicates that the optimal stationary relational contract form is related to discount factor of the two parties，the probability of continuous cooperation of the two parties，and the ex-ante and ex-post kept return of the farmers that the optimal incentive plan is strictly depending on the payment to the farmers by the leading enterprises and the contrast relation between ex-ante kept return utility and ex-post kept return utility and that this contrast relation makes the operation efficiency of relational contract of agricultural products trading not related to surplus return distribution situation any more.