Taken a domestic offshore platform as an example, the flare arm length under different flare arm angles and wind speeds is systematically analyzed from three aspects of thermal radiation, noise and diffusion by Flaresim and Phast Software. It is concluded that: the thermal radiation is the key factor to determine the flare arm length; the flare arm angle directly affects the length of the flare arm, for the flare system of the target platform, the determining operating point changes from the flare arm root (0°-30°) to the monkey board of the workover rig (45°- 90°); 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. The research results can provide reference for the preliminary design of the flare system.