Focusing on the ignorance of the flexibility of the agent's effort in decision, this paper biulds a model of agent's flexible effort decision model and a model of principle's design of sharecropping contract in the case of complete period-0 information, manages to prove the existence of the optimal sharecropping fraction, and compares the conclusions with those derived from the classical principle-agent models. The results show that the flexibility of an agent's effort in decision raises the threshold value...