Capacity is an important technical and economic indicator to evaluate the production and operating state of steel plants. Aiming at the problems about the capacity of steelmaking-continuous casting section, firstly, the operation time of steelmaking, refining and continuous casting process for different steel grades is found out by resolving the production process of a BOF special steel plant. Then, the bottleneck process under different production mixes is found by discussing the relation among the productive capacity of steelmaking-continuous casting system, the capacity of different processes and the products mixes．Finally，the improvement measures for promoting the capacity are proposed aiming at breaking through the restriction of bottleneck process. A general method to research on the capacity of steelmaking-continuous casting section is developed, which helps to determine reasonable index of products mix and give full play to the capacity of steelmaking-continuous casting section.