In a reactor for solid ammonium salt, a method of controlling the reactor, and a NOx emission purification system using solid ammonium salt and selective catalytic reduction, the reactor includes a first chamber and a second chamber. The first chamber has an exhaust and a first heating element. Solid ammonium salt is in the first chamber. The second chamber has a second heating element and is formed at a side of the first chamber. The first chamber is connected with the second chamber. Solid ammonium salt is in the second chamber. An amount of the solid ammonium salt in the second chamber is more than that in the first chamber, so that the first chamber is heated and cooled faster than the second chamber.