Abstract: With the gradually developing of all types of new rural financial organizations, the regulators have been more pressure, asked to relax control and expand them financing ability. But as international experience showed, following the growth financial capacity, this kind of financial organization have to deviate or give up the aim of servicing the farmers and small and medium enterprises. On the base of private information, a limited supply theory model is constructed in this paper. It can be used to answer the question of what is the optimal financial scale of this kind of organization. The conclusions as following: each of them can only service limited vulnerable group, have only limited supply capacity; with excessive increase of financial ability, this kind of organization will reduce the size of the loan service to vulnerable groups, such as ‘SanNong’; in contrast, the direct selling is better than the underwriting on bond financing, because the direct selling can help to improving the financial coverage; because of the lack of private information ,if the financing objects are restrained, the bond financing will be very difficult.