The length of flare arm of offshore platform affects the safety, structural center of gravity and engineering investment of the platform. In the design of the flare system, the flare arm angle directly affects the determination of the flare arm length. Taking a domestic offshore platform as an example, the flare arm length under different angles and wind speeds is systematically analyzed from three aspects of thermal radiation, noise and diffusion by using FLARESIM and PHAST Software. It is concluded that: ① Thermal radiation is the key factor to determine the flare arm length; ② The flare arm angle directly affects the length of the flare arm. For the flare system of a domestic platform, it determines that the operating point changes from the flare arm root (0° to 30°) to the monkey board of the workover rig (45° to 90°); ③ When the flare arm angle is 45°, the flare arm length can be minimized and the safety and economy of the platform can be considered.