After the temperature medium voltage coal boiler changed to burn biomass, the quantity of the smoke greatly increased, the economizer heat increased and the boiling rate rose, which tends to cause the economizer’s tuber wall to overheat. In this article, the calculation formula of the economizer’s tube’s outside surface temperature and the average temperature are deduced based on the basic thermal law. Besides, by measuring the economizer’s wall temperature, the relation between the wall temperature and the boiling rate is attained and two limit boiling rate concepts which judge the safe work of the economizer are also put forward. What’s more, the window procedure which calculates the wall temperature and the limit boiling rate is also compiled. In the end, the authors summarize the rapid- increasing change rule of the wall temperature between the two limit boiling rates and hold that the biomass boiler boiling rate should be controlled below the first boiling rate.