The invention relates to a method for hot melting and aluminizing large steel, which comprises the following steps: the steel is pretreated, and the pretreatment includes decreasing, rust removing and washing; the pretreated steel is processed of assistant plating treatment so as to form an anti-oxidation and passivation film; the steel processed of assistant plating treatment is preheated so as to slowly heat up the steel to not more than 300 DEG C, and the preheated steel is processed of salt bath treatment. The composition of the weight percentage of adopted fused salt is as follows: 38 percent of NaCl, 30 percent of KCl, 20 percent of Na3AlF6, 38 percent of AlF, 2 percent of CaCl and 2 percent of MgF2. The steel after salt bath treatment is processed of hot melting and aluminizing treatment; and the steel after hot melting and aluminizing treatment is processed of thermal insulating and slow cooling treatment. The invention not only increases the activating and infiltrating (alloying) function of aluminum to the steel surface in the process of melting and aluminizing so that the alloy layer (composite metal material of aluminum and metal) is more compact and uniform, but also eliminates the thermal stress of the steel to the maximum so as to minimize the steel deformation.