A flue gas mixing device and method. The device comprises: a flue inlet section (11); a throat section (22); a flue outlet section (12); a narrowing section (21); and an expanding section (23). The flue inlet section (11), the throat section (22) and the flue outlet section (12) are communicated in sequence. A plurality of air inlets (221) are arranged around a sidewall of the throat section (22). An enclosure cavity (3) is arranged at the periphery of the throat section (22). The enclosure cavity (3) is connected to a circular cooler exhaust branch pipe (31). The narrowing section (21) is connected between the flue inlet section (11) and the throat section (22). The expanding section (23) is connected between the throat section (22) and flue outlet section (12).