Small sum credit is a efficient way to help farmer shake off poverty. The opportunity cost of win the loan, refinancing rat and the probability of success of have effects on the development of this item. In many areas, if the loan can't be paid in time, farmer will be punished. The way has great efficiency on the farmer who owes the loan on purpose. But the effect on the farmer who doesn't owe the loan on purpose of the way hasn't been studied. The relationship between the probability of winning the loan and these factors has been studied by a simple method in this paper. The conclusion is, the decrease in the opportunity cost and refinancing rat, the increase in the probability of success can help farmer win the loan. If there is no outstanding payment on purpose, the punishment has no advantage.