The invention relates to the field of fine chemicals, and specifically relates to a method for preparing (chloromethylene) dimethylammonium chloride through a phased reaction. According to the method, 2-methyl-1-propanol and water are adopted as a mixed solvent; an equal molar amount of thionyl chloride is dropped in N,N-dimethylformamide, and a reaction is allowed; sulfur dioxide is removed through reduced pressure, and (chloromethylene) dimethylammonium chloride is prepared. The reaction is divided into three phases according to the following steps: (1) a constant-temperature reaction is carried out for 1h under a temperature of 35 DEG C; (2) an increased-temperature reaction is carried out, wherein the temperature is increased to 45 DEG C with a heating speed of 0.5 DEG C/min, and a reaction is carried out for 0.5h; the temperature is increased to 50 DEG C with a heating speed of 1 DEG C/min, and a reaction is carried out for 0.5h; and (3) a reflux reaction is carried out, wherein the temperature is increased, and the reflux reaction is carried out for 2h. In the (chloromethylene) dimethylammonium chloride provided by the invention, SO2 content is no higher than 0.5%. During an application process, basically no waste gas is produced, such that the requirement of environment protection is satisfied. During the production process, the produced SO2 can be recovered and applied in thionyl chloride production, such that circular economy is realized.