New and useful method of installing furnace walls for a boiler that does not require dedicated temporary lifting mechanism and reduces the installation work period significantly is disclosed. The method of installing furnace walls for a boiler configured with an upper portion of boiler proper 30 and a lower portion of boiler proper 40, temporally suspending two left and right face furnace water wall panels 48 among four furnace water wall panels, the upper portion of boiler proper is assembled while it is hanged from a top girder module 28 positioned on the top portion of a center portion in a boiler frame, a furnace rear wall 44 and a furnace front wall 42 among the furnace water wall panels are hanged sequentially, the temporally suspended two left and right face furnace water wall panels 48 are moved to designated positions, and the upper ends of the furnace water wall panels 48, 44 and 42 are connected with the lower end of the upper portion of boiler proper 30.