The invention relates to the technical field of gas heat exchange and condensing equipment, in particular to a crystallizer. The crystallizer comprises at least two heat exchange units and condensing tubes, wherein the heat exchange units are arranged successively according to the process sequence, namely, the heat exchange units are arranged in turns; every two adjacent heat exchange units are communicated with each other through at least one condensing tube; a gas inlet is formed in the first heat exchange unit; a gas outlet is formed in the last heat exchange unit; a discharge opening is formed in the bottom of each heat exchange unit. The heat exchange units of the crystallizer are connected through the condensing tubes, and heat exchange travel and area are increased; moreover, the condensing tubes are tubular, pressure difference exists between the condensing tubes and gas entering the condensing tubes from the heat exchange units, gas flow speed can be increased, and therefore, heat exchange efficiency is improved. The heat exchange units are connected by the condensing tubes to form a whole body which is serpentine, gas continuously flows inside the crystallizer and is high in speed, so that crystallization on tube walls stops after the thicknesses of crystals reach to a certain degree, and the crystals can be controlled uniformly.