In this study, an optimization method is proposed based on JIT, to divide station group and arrange the distribution vehicles. In order to eliminate case that assembly line production process was shut down due to material shortage, with the capacity of distribution vehicles and distribution timewindow as the constraint, a model is developed, taking the full load ratio of distribution vehicles and station material requirement urgency as the optimization goal. Genetic algorithm is used to solve the model to optimize the total number of daily deliveries, which consequently give the plan of distribution of work station groups and vehicle scheduling. At last, the feasibility and effectiveness of this method was proved by the verification of example. Key words： mixedflow assembly line; material distribution; Justintime(JIT); station groups division; full load ratio; genetic algorithm