Abstract: The large-scale integration of clean energy into the power grid generates significant demands for flexibility in the distribution network, hereafter, the transmission congestion of the flexibility resources needs to be urgently addressed. Therefore, this paper, by exploring the transmission principles of the flexibility resources in the distribution network, constructs a flexibility resource network transmission model. Considering the distribution network flow, a day-ahead optimization method for the distribution network is proposed that takes into account the flexibility resource transmission constraints. Firstly, a flexibility analysis framework for the distribution network is established by considering both the node flexibility and the network flexibility. Secondly, the flexibility demand and the resource models for each of the nodes are constructed. Then, the virtual power flow is used to build the network transmission flexibility resource model in order to describe the coupling relationship between the flexible resource transmission and the distribution network power flow. The model quantifies the transmission capacity of the flexibility resources by considering the transmission restrictions of the node voltage and the line transmission capacities. Finally, an optimization strategy for the distribution network that considers the flexibility resource transmission constraints is proposed. The case study demonstrates that the proposed flexibility analysis method is aligned with the operating characteristics of the distribution network, effectively avoids the transmission congestion of the flexibility resources in the actual operation, reduces the operating cost of the distribution network, and improves the clean energy consumption rate.