Why can I board the plane at Xiaoshan Airport in yellow code?

You can't. When the health code is yellow, you cannot fly. Because the health code yellow code will have a certain risk of infection, when the health code is yellow code, you must abide by the relevant epidemic prevention regulations. Those who travel without authorization shall be punished in violation of the law on public security administration punishment. So it is best not to know the law and break the law.