Multi-period allocation of machines and labours in flexible job-shop is studied based on cost. Comprehensively considering the machine processing cost, labor cost, cost of workpiece transportation, inventory cost, backorder cost and outsource cost, a dynamic manufacturing resources allocation model is established. Meanwhile, the solution of the proposed model is designed based on the genetic algorithm. A case where 10 machines and 10 workers are allocated to manufacture 5 different types of products during 3 manufacturing periods is analysed, and the optimal allocation plan is figured out. The result verifies the effectiveness of the proposed model and algorithm.