The invention relates to a modification furnace connected with a gasifier. The modification furnace comprises a furnace body, a modification assembly and multiple temperature sensors. The lower part of the furnace body is provided with a destructive distillation gas inlet and an ash outlet, and the upper part of the furnace body is equipped with a modified gas outlet. The modification assembly comprises multiple first heating air conduits arranged inside the furnace body, multiple second heating air conduits and multiple support plates. The multiple heating air conduits which are placed in the same horizontal plane are arranged parallel to each other. The multiple second heating air conduits are vertically arranged between adjacent two first heating air conduits, and two ends of each second heating air conduit are interconnected with adjacent two first heating air conduits. Each support plate is glued on each first heating air conduit. A thermal storage agent is filled on the support plates. The multiple temperature sensors are arranged at different positions of the thermal storage agent. According to the invention, impurities or the thermal storage agent in modified gas are avoided from blocking the heating air conduits, and modification effect of the destructive distillation gas is improved.