The invention relates to a method for producing a low-activation martensitic steel for a fusion reactor by melting. The method comprises: step one, a charging material is smelted in a 
vacuum induction furnace: (1) when a vacuum degree is between 0.1 and 10 Pa, 
argon gas is passed through the 
vacuum induction furnace till the vacuum degree is between 0.005 and 0.08MPa, and a 
smelting temperature is between 1450 and 1650 DEG C; (2) alloys are added in sequence: firstly, pure Fe is melted; Cr and W are added into the furnace; then Ta and V are added into the furnace, and finally C and Mn are added into the furnace; simultaneously, the level of an 
oxygen content in the pure Fe is observed; before Cr is added, a little of C is added into the furnace for carrying out deoxidation treatment; (3) on-line detection is carried out to the compositions of 
alloy elements; the 
alloy elements are added according to a detection result; and (4) after the compositions are qualified, a mixture is cast under the vacuum at a 
casting temperature of between 1450 and 1550 DEG C. Step two, according to the compositions and the inner 
quality level of a cast 
ingot which is obtained by vacuum induction and melting, whether the cast 
ingot is remelted and refined in a vacuum electroslag remelting furnace or a common electroslag remelting furnace with 
argon protection. Step three, a needed CLAM 
steel bar material is prepared by the production of 
forging, cogging and hot rolling. The method has the characteristics of simple 
smelting operation, stable process and low production cost; the compositions and impurities of the produced CLAM steel meet the design requirement; and the method has no aliquation of the compositions and tissues and high degree of purity and is suitable for the application of the fusion reactor.