The invention discloses a room-temperature rust removal blackening agent and a preparation method thereof, and belongs to the technical field of metal surface anti-corrosion treatment. The rust removal blackening agent is prepared from a rust removal solution, a blackening solution, a composite emulsion, nano-montmorillonite, a polyaniline emulsion, texanol ester alcohol, glycerinum and deionized water. The method comprises the following steps: preparing the rust removal solution, the blackening solution and the composite emulsion, and then respectively adding the nano-montmorillonite, the glycerinum and the polyaniline emulsion to the specified amount of deionized water; adding the rust removal solution under a continuous stirring condition, stirring evenly, and slowly adding the blackening solution; further stirring and dispersing, and then slowly adding the composite emulsion; and slowly adding the texanol ester alcohol and further stirring under a room temperature condition, so as to prepare the room-temperature rust removal blackening agent. The room-temperature rust removal blackening agent disclosed by the invention can directly carry out rust removal and blackening on the steel surface to replace the technologies of acid pickling, rust removing and the like. The room-temperature rust removal blackening agent has the characteristics of high rust removal blackening speed, excellent antiseptic property and the like.