Belonging to the field of molecular sieve synthesis, the invention relates to a method for preparing a 3A type zeolite molecular sieve. The preparation method of a 3A type zeolite molecular sieve is characterized by comprising the steps of: 1) calcination of kaolin, thus obtaining metakaolin; 2) preparation of a 4A molecular sieve: mixing metakaolin and a NaOH aqueous solution uniformly, thus obtaining a mixture; filtering the mixture, washing the filter cake with distilled water, and conducting filtration again, drying the filter cake from second filtration at a temperature of 90-100DEG C for 2-4h, thus obtaining the 4A molecular sieve; 3) preparation of a 3A type zeolite molecular sieve: reacting the 4A molecular sieve with a KCl solution in a reaction vessel for 3-5h at a temperature of 70-90DEG C, then conducting cooling, washing, and filtering, drying the obtained filter cake at a temperature of 90-100DEG C for 2-4h, thus obtaining the 3A type zeolite molecular sieve. The method of the invention has simple process and low calcination temperature, and the prepared 3A type zeolite molecular sieve has great calcium ion exchange capacity and good performances.