Abstract: To address the challenges posed by the uncertainty of offshore wind power output to power system frequency security and transmission network planning, this paper proposes a transmission-storage coordinated robust planning method considering analytical dynamic frequency constraints. First, a bi-level optimization framework is formulated, consisting of upper-level transmission network planning and lower-level operation scheduling; the upper level aims to minimize the total system investment and operation cost while planning transmission lines that satisfy load rate constraints, and the lower level considers the coordinated operation of the transmission network and energy storage system (ESS) with frequency response modeling to guarantee dynamic frequency security under disturbances. Then, the bi-level model is transformed into a tractable single-level model via strong duality theory. Furthermore, an adversarial scenario generation (ASG) method is employed to construct extreme risk scenarios, the probabilistic uncertainty of offshore wind power and load is characterized by a 1-∞ norm hybrid uncertainty set, and conditional value-at-risk (CVaR) is adopted to quantify the corresponding risk loss, based on which a transmission-storage coordinated distributionally robust optimization model is established and solved by the column-and-constraint generation (C&CG) algorithm. Simulation results on the IEEE 24-bus system demonstrate that the proposed method can enhance system frequency security, effectively alleviate line overloading, and yield planning schemes with favorable economic efficiency and robustness.