It has been trouble in popularizing small agricultural machinery because there is contradiction between multi-function and low price. Modular design under mass customization is an effective method to solve this problem, however, the effect of design has been influenced by the rationality of module partition. A module partition approach based on interface restraints that were described quantitatively by fuzzy membership function was proposed by fuzzy graph and cut-sets. An example of series SF5 machine adopted modular design showed that combination with different modules could meet various functional requirements of the users.