The existing large amount of sulfate in brine has a huge effect on carnallite precipitation during the process of producing carnallite from salt field. A series of experiments of effect of different sulfate concentration on carnallite quality precipitated from brine were conducted. The results indicated that sulfate in brine could have a huge effect on carnallite quality at the condition when sulfate content were at a high level or evaporating brine at high temperature. In addition, the content of sulfate in solid products at the carnallite precipitation stage is the highest when sulfate content is 2% in the simulated brine. Results from laboratory experiments show that sulfate has a certain effect on producing carnallite, it have some degree of guiding significance in process optimization of producing carnallite.