The digital twin of flexible production line can realize the realtime perception of the operation state of flexible production process, and optimize production by using twin data. For this reason, a state aware method of flexible production line based on digital twin is proposed. The architecture of the method is established firstly, and three key technologies in the system implementation are discussed in detail: the construction of digital twin model based on unity 3D, the realtime acquisition of heterogeneous equipment data based on OPC UA, and the state perception and evaluation based on twin model. Finally, based on a flexible production workshop, the state perception and fault diagnosis are realized, and the feasibility and effectiveness of the proposed method are verified, which provides an effective solution for the realization of the state perception of the flexible production line.