The invention discloses a rust inhibitor for a steel bar for an autoclaved sand aerated concrete board and a preparation method. The rust inhibitor is prepared from the following components in percentage by weight: 53 to 57 percent of carboxylic butadiene-styrene latex, 1.2 to 1.7 percent of iron oxide red, 8 to 11 percent of slaked lime, 3.2 to 3.7 percent of high-efficiency thickener, 18 to 22 percent of quartz sand, 5 to 10 percent of sand aerated concrete board waste material and 3 to 7 percent of tap water. Raw materials of the rust inhibitor provided by the invention are easily purchased, and the rust inhibitor has a low cost and a simple confecting process, can be abundantly confected once to be stored and used, has high adhesive force with the steel bar and is environmentally friendly and pollution-free.