The invention discloses an underground gasification furnace and method of coal. The underground gasification furnace of coal comprises multiple gasification units. Each gasification unit comprises multiple vertical wells and at least two horizontal directional wells. The vertical wells in the same gasification unit are communicated with one another and the horizontal directional wells are communicated with at least one vertical well. At least one horizontal directional well is a ventilated well and all the vertical wells in the same gasification unit serve as gas outflow wells jointly. The arrangement modes of the vertical wells and the horizontal directional wells are reasonably set, unit combinations are composed of the two or more horizontal directional wells and the vertical wells, the underground gasification furnace capable of gasifying a large coal seam is formed, and construction of directional wells of a gas outflow column is avoided. In the unit gasifying process, the structural units on the two sides can be gradually constructed, furnace construction time is shortened, and economic benefits are improved.