A non-formalization description model based on the bipartite graph was established to express the key factors and the matching relationship between the parts. The model could also make individual changes easily. According to proportion of the parts' quantity, the selective assembly could be divided into two kinds of matching problem, which were 1-matching and d-matching. The restrictions for d-matching were defined. Making use of a 0-1 matrix, the non-formalization- description was transformed to a formalization model which was easy to be calculated by the algorithm. On this base, a genetic algorithm was put forward for the model, and a case of selective assembly was applied to prove the model.