Abstract: This paper presents the physical design and simulation of a 210 GHz, megawatt (MW)-level, long-pulse gyrotron for electron cyclotron resonance heating (ECRH) in next-generation magnetic confinement fusion devices. The tube utilizes an ultra-high-order TE_34,12 mode cavity. Driven by an 80 kV, 40 A electron beam, the nonlinear self-consistent simulation predicts an output power of 1.08 MW with an electronic efficiency of 33.8% and an overall efficiency of 54% with a 30 kV depressed collector. Furthermore, the design and simulation of key components, including a cavity, a quasi-optical mode converter, a triode-type electron gun, combined vertical and transverse magnetic sweeping coils and a chemical vapor deposition diamond output window are detailed. Thermal analysis of the water-cooled cavity and collector confirms the tube’s capability for long-pulse continuous-wave operation. The results demonstrate that the proposed 210 GHz gyrotron design meets the key technical requirements for next-generation ECRH systems operating above 200 GHz.